TestRuntime.FX46's output shouldn't be referenced

src/Test/Utilities/Portable/TestUtilities.csproj

@@ -34,6 +34,8 @@
     <ProjectReference Include="..\Runtime.FX46\TestRuntime.FX46.csproj">
       <Project>{23683607-168A-4189-955E-908F0E80E60D}</Project>
       <Name>TestRuntime.FX46</Name>
+      <!-- TestRuntime.FX46 exists only to have a single project copying binaries into the output directory. We should not reference it's output -->
+      <ReferenceOutputAssembly>false</ReferenceOutputAssembly>
     </ProjectReference>
     <ProjectReference Include="..\..\..\Compilers\Test\Resources\Core\CompilerTestResources.csproj">
       <Project>{7fe6b002-89d8-4298-9b1b-0b5c247dd1fd}</Project>