Move shared tests to VS Code project (#12328)

Copilot seems very reluctant to read our instructions file, and loves
running dotnet test on the specific project that it made/modified tests
in. A lot of the cohosting tests are shared between VS and VS Code, so
will happily run on the Linux images that Copilot uses, but because the
physical files are in the VS project, every time Copilot tries to run
them, it can't because thats a .NET Framework project.

Eventually when we clean up the LSP and legacy editors we can move
things around and stop using shared files, but until then having the
files physically in the VS Code project, and shared across to VS, rather
than the opposite, should hopefully mean Copilot falls into the pit of
success a little more easily.

Author: davidwengier

src/Razor/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/Microsoft.VisualStudio.LanguageServices.Razor.Test.csproj

@@ -14,6 +14,8 @@
     <Compile Include="..\Microsoft.CodeAnalysis.Razor.Workspaces.Test\Shared\**\*.cs">
       <Link>Shared\%(RecursiveDir)%(FileName)%(Extension)</Link>
     </Compile>
+
+    <Compile Include="..\Microsoft.VisualStudioCode.RazorExtension.Test\Endpoints\Shared\*.cs" LinkBase="Cohost\Shared" />
   </ItemGroup>
 
   <ItemGroup>