Get integration tests running again (#10285)

Restores the reference to `xunit.runners.visualstudio` that was
inadvertently lost when moving to CPM.

Run is happening now, but signs are positive:
https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=9450237

src/Razor/test/Microsoft.VisualStudio.Razor.IntegrationTests/Microsoft.VisualStudio.Razor.IntegrationTests.csproj

@@ -36,6 +36,17 @@
     <PackageReference Include="Microsoft.Internal.VisualStudio.Shell.Framework" />
     <PackageReference Include="xunit" />
     <PackageReference Include="xunit.runner.utility" />
+
+    <!--
+      We need to reference XUnit, but Arcade won't import it because the test runner above is MSTest,
+      and we can't just reference the package we need because they are implicit references in Arcade
+      for other projects, and CPM doesn't play nicely with those. We could try to manually import
+      the Arcade XUnit.targets file, but finding that file is a pain, so instead we can just manually
+      import the package ourselves, but as an implicit definition to stop Nuget complaining.
+      TL;DR: MSBuild was designed to cause pain.
+    -->
+    <PackageReference Include="xunit.runner.console" Version="$(XUnitRunnerConsoleVersion)" IsImplicitlyDefined="true" PrivateAssets="all" Publish="true"/>
+    <PackageReference Include="xunit.runner.visualstudio" Version="$(XUnitRunnerVisualStudioVersion)" IsImplicitlyDefined="true" PrivateAssets="all" Publish="true"/>
   </ItemGroup>
 
   <ItemGroup>