Revert the change in test\TestAssets\TestProjects\XUnitTestProject\XUnitTestProject.csproj file and use the fixed version of the package.

Author: Simon Zhao (BEYONDSOFT CONSULTING INC)

test/TestAssets/TestProjects/XUnitTestProject/XUnitTestProject.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="$(MicrosoftNETTestSdkPackageVersion)" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0" />
     <PackageReference Include="xunit" Version="2.2.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0" />
   </ItemGroup>