Simplify the app.config model for testing

build/Defaults/Net461/app.config

-<?xml version="1.0" encoding="utf-8"?>
-
-<!-- The default app.config for test projects that do not specify one -->
-<configuration>
-  <appSettings>
-    <add key="xunit.diagnosticMessages" value="false"/>
-    <add key="xunit.parallelizeTestCollections" value="false"/>
-    <add key="xunit.shadowCopy" value="false"/> <!-- Set shadow copy to false so that the VS Test Explorer can properly load our public signed binaries -->
-  </appSettings>
-  <system.diagnostics>
-    <trace>
-      <listeners>
-        <remove name="Default" />
-        <add name="ThrowingTraceListener" type="Microsoft.CodeAnalysis.ThrowingTraceListener, Roslyn.Test.Utilities" />
-      </listeners>
-    </trace>
-  </system.diagnostics>
-</configuration>

build/Defaults/Net462/app.config

-<?xml version="1.0" encoding="utf-8"?>
-
-<!-- The default app.config for test projects that do not specify one -->
-<configuration>
-  <appSettings>
-    <add key="xunit.diagnosticMessages" value="false"/>
-    <add key="xunit.parallelizeTestCollections" value="false"/>
-    <add key="xunit.shadowCopy" value="false"/> <!-- Set shadow copy to false so that the VS Test Explorer can properly load our public signed binaries -->
-  </appSettings>
-  <system.diagnostics>
-    <trace>
-      <listeners>
-        <remove name="Default" />
-        <add name="ThrowingTraceListener" type="Microsoft.CodeAnalysis.ThrowingTraceListener, Roslyn.Test.Utilities" />
-      </listeners>
-    </trace>
-  </system.diagnostics>
-</configuration>

build/Targets/Roslyn.Toolsets.Xunit.targets

@@ -54,8 +54,7 @@
   <Target Name="AddDefaultTestAppConfig">
     <PropertyGroup>
       <_AlreadyHasAppConfig Condition="'%(None.Filename)%(None.Extension)' == 'app.config'">true</_AlreadyHasAppConfig>
-      <_DefaultAppConfigFile Condition="'$(TargetFramework)' == 'net46'">$(MSBuildThisFileDirectory)..\Defaults\Net460\app.config</_DefaultAppConfigFile>
-      <_DefaultAppConfigFile Condition="'$(TargetFramework)' == 'net461'">$(MSBuildThisFileDirectory)..\Defaults\Net461\app.config</_DefaultAppConfigFile>
+      <_DefaultAppConfigFile Condition="'$(TargetFramework)' != 'netcoreapp2.0'">$(MSBuildThisFileDirectory)..\Defaults\Desktop\app.config</_DefaultAppConfigFile>
       <_DefaultAppConfigFile Condition="'$(TargetFramework)' == 'netcoreapp2.0'">$(MSBuildThisFileDirectory)..\Defaults\Portable\app.config</_DefaultAppConfigFile>
     </PropertyGroup>