Fix netcoreapp2.0 use in TestUtilities

The Roslyn code base defaults to enabling the
`<DisableImplicitFrameworkReferences>` property and instead specifies manual
references. This is mostly an artifact of our code base evolution from
the project.json days where implicit references weren't an option.

This means that even though TestUtilities specified netcoreapp2.0 as a
TF it was still mostly targeting netstandard1.3. This is because
implicit references weren't expanded and we were manually adding only a
subset of the available packages.

Enabled implicit references here, conditioned the explicit ones to
netstandard1.3 and now we can remove a number of `#if` uses.

closes #25228

src/Compilers/CSharp/Test/Emit/Attributes/EmitTestStrongNameProvider.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-// The use of SigningTestHelpers makes this necessary for now. Fix is tracked by https://github.com/dotnet/roslyn/issues/25228
-#if NET461
 using System;
 using System.Collections.Immutable;
 using System.IO;
@@ -163,4 +161,3 @@ class C
         }
     }
 }
-#endif

src/Test/Utilities/Portable/Platform/Custom/MetadataSignatureHelper.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
-#if NET461 || NET46 || NETCOREAPP2_0
+#if !NETSTANDARD1_3
 using System;
 using System.Collections.Generic;
 using System.Globalization;
@@ -542,14 +542,10 @@ static private void AppendFieldInfo(FieldInfo field, StringBuilder sb)
 
             if (field.IsInitOnly) sb.Append("initonly ");
             if (field.IsLiteral) sb.Append("literal ");
-#if NET46 || NET461
             if (field.IsNotSerialized) sb.Append("notserialized ");
-#endif
             if (field.Attributes.HasFlag(FieldAttributes.SpecialName)) sb.Append("specialname ");
             if (field.Attributes.HasFlag(FieldAttributes.RTSpecialName)) sb.Append("rtspecialname ");
-#if NET46 || NET461
             if (field.IsPinvokeImpl) sb.Append("pinvokeimpl ");
-#endif
 
             sb.Append(field.IsStatic ? "static " : "instance ");
             AppendType(field.FieldType, sb);

src/Test/Utilities/Portable/Platform/Desktop/SigningTestHelpers.cs → src/Test/Utilities/Portable/Platform/Custom/SigningTestHelpers.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-#if NET461 || NET46
+#if !NETSTANDARD1_3
 using System;
 using System.Collections.Immutable;
 using System.IO;

src/Test/Utilities/Portable/TestUtilities.csproj

@@ -12,6 +12,7 @@
     <DefineConstants>$(DefineConstants);DNX</DefineConstants>
     <NoStdLib>true</NoStdLib>
     <RootNamespace></RootNamespace>
+    <DisableImplicitFrameworkReferences>false</DisableImplicitFrameworkReferences>
   </PropertyGroup>
   <ItemGroup Label="Project References">
     <ProjectReference Include="..\..\..\Compilers\Test\Resources\Core\CompilerTestResources.csproj" />
@@ -95,36 +96,15 @@
     <PackageReference Include="Microsoft.DiaSymReader.Converter.Xml" Version="$(MicrosoftDiaSymReaderConverterXmlVersion)" />
     <PackageReference Include="Microsoft.Metadata.Visualizer" Version="$(MicrosoftMetadataVisualizerVersion)" />
     <PackageReference Include="Microsoft.NETCore.Platforms" Version="$(MicrosoftNETCorePlatformsVersion)" />
-    <PackageReference Include="Microsoft.Win32.Primitives" Version="$(MicrosoftWin32PrimitivesVersion)" />
-    <PackageReference Include="System.AppContext" Version="$(SystemAppContextVersion)" />
-    <PackageReference Include="System.Collections.Concurrent" Version="$(SystemCollectionsConcurrentVersion)" />
-    <PackageReference Include="System.Console" Version="$(SystemConsoleVersion)" />
-    <PackageReference Include="System.Data.Common" Version="$(SystemDataCommonVersion)" />
-    <PackageReference Include="System.Diagnostics.Debug" Version="$(SystemDiagnosticsDebugVersion)" />
-    <PackageReference Include="System.Diagnostics.Tools" Version="$(SystemDiagnosticsToolsVersion)" />
+    <PackageReference Include="xunit" Version="$(xunitVersion)" />
+    <PackageReference Include="xunit.analyzers" Version="$(xunitanalyzersVersion)" />
+    <PackageReference Include="xunit.runner.console" Version="$(xunitrunnerconsoleVersion)" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard1.3'">
+    <PackageReference Include="System.Reflection.TypeExtensions" Version="$(SystemReflectionTypeExtensionsVersion)" />
     <PackageReference Include="System.Diagnostics.TraceSource" Version="$(SystemDiagnosticsTraceSourceVersion)" />
     <PackageReference Include="System.Diagnostics.Process" Version="$(SystemDiagnosticsProcessVersion)" />
-    <PackageReference Include="System.IO.FileSystem" Version="$(SystemIOFileSystemVersion)" />
-    <PackageReference Include="System.Net.Primitives" Version="$(SystemNetPrimitivesVersion)" />
-    <PackageReference Include="System.Reflection.TypeExtensions" Version="$(SystemReflectionTypeExtensionsVersion)" />
-    <PackageReference Include="System.Security.Cryptography.Algorithms" Version="$(SystemSecurityCryptographyAlgorithmsVersion)" />
-    <PackageReference Include="System.Text.RegularExpressions" Version="$(SystemTextRegularExpressionsVersion)" />
     <PackageReference Include="System.Threading.Thread" Version="$(SystemThreadingThreadVersion)" />
-    <PackageReference Include="System.Xml.XDocument" Version="$(SystemXmlXDocumentVersion)" />
     <PackageReference Include="System.Xml.XmlDocument" Version="$(SystemXmlXmlDocumentVersion)" />
-    <PackageReference Include="System.Runtime.Loader" Version="$(SystemRuntimeLoaderVersion)" Condition="'$(TargetFramework)' == 'netcoreapp2.0'" />
-    <PackageReference Include="xunit" Version="$(xunitVersion)" />
-    <PackageReference Include="xunit.analyzers" Version="$(xunitanalyzersVersion)" />
-    <PackageReference Include="xunit.runner.console" Version="$(xunitrunnerconsoleVersion)" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="$(xunitrunnervisualstudioVersion)" />
-
-    <!-- This reference is necessary to pull down the assemblies we compile + run against 
-         in the compiler layer when running on CoreClr. Don't actually want to reference 
-         anything here, just have it available -->
-    <PackageReference Include="NETStandard.Library" Version="$(NETStandardLibraryFixedVersion)" Condition="'$(TargetFramework)' == 'netcoreapp2.0'">
-      <PrivateAssets>all</PrivateAssets>
-      <ExcludeAssets>all</ExcludeAssets>
-    </PackageReference>
   </ItemGroup>
-  <ProjectExtensions />
 </Project>