revert TFM changes

src/Microsoft.CodeAnalysis.Testing/Directory.Build.props

@@ -16,7 +16,7 @@
       netstandard1.5/net46: Roslyn 2.x
       netstandard2.0/net472: Roslyn 3.x
     -->
-    <TestingLibraryTargetFrameworks>netcoreapp3.1;netstandard2.0;net472</TestingLibraryTargetFrameworks>
+    <TestingLibraryTargetFrameworks>netcoreapp3.1;netstandard1.5;netstandard2.0;net452;net46;net472</TestingLibraryTargetFrameworks>
 
     <!--
       netcoreapp3.1: Ensures full support for nullable reference types

tests/Microsoft.CodeAnalysis.Testing/Directory.Build.props

@@ -21,7 +21,7 @@
   <Import Project="$([MSBuild]::GetPathOfFileAbove('Directory.Build.props', '$(MSBuildThisFileDirectory)../'))" />
 
   <PropertyGroup>
-    <TestTargetFrameworks>netcoreapp3.1;net472</TestTargetFrameworks>
+    <TestTargetFrameworks>netcoreapp3.1;net472;net46;netcoreapp1.1;netcoreapp2.0</TestTargetFrameworks>
     <SourceGeneratorTestTargetFrameworks>netcoreapp3.1;net472</SourceGeneratorTestTargetFrameworks>
 
     <!-- Workaround dependencies that do not yet support netcoreapp3.1 https://github.com/dotnet/roslyn/issues/45114 -->