Add a NuGet.Config to stabilize tests

This change prevents the customized NuGet.Config from the Roslyn repository from
being used by Visual Studio when integration tests are running in CI builds.

build/scripts/build.ps1

@@ -725,6 +725,7 @@ function Redirect-Temp() {
     Copy-Item (Join-Path $repoDir "src\Workspaces\CoreTestUtilities\Resources\Directory.Build.props") $temp
     Copy-Item (Join-Path $repoDir "src\Workspaces\CoreTestUtilities\Resources\Directory.Build.targets") $temp
     Copy-Item (Join-Path $repoDir "src\Workspaces\CoreTestUtilities\Resources\Directory.Build.rsp") $temp
+    Copy-Item (Join-Path $repoDir "src\Workspaces\CoreTestUtilities\Resources\NuGet.Config") $temp
     ${env:TEMP} = $temp
     ${env:TMP} = $temp
 }

src/Workspaces/CoreTestUtilities/Resources/NuGet.Config

+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <!-- Provide a default package restore source for test projects. -->
+  <packageRestore>
+    <add key="enabled" value="true" />
+  </packageRestore>
+  <packageSources>
+    <clear />
+    <add key="api.nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
+    <add key="dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" protocolVersion="3" />
+  </packageSources>
+</configuration>