Enable tests in VMR

[NikolaMilosavljevic]

Fixes #76960

Roslyn tests are failing in VMR with mismatched package version errors, i.e.:

C:\git\dotnet\src\roslyn\src\Compilers\Core\CodeAnalysisTest\Microsoft.CodeAnalysis.UnitTests.csproj : error NU1109: Detected package downgrade: Microsoft.TestPlatform.ObjectModel from 17.12.0 to centrally defined 17.5.0. Update the centrally managed package version to a higher version.  [C:\git\dotnet\src\roslyn\Roslyn.sln]
C:\git\dotnet\src\roslyn\src\Compilers\Core\CodeAnalysisTest\Microsoft.CodeAnalysis.UnitTests.csproj : error NU1109:  Microsoft.CodeAnalysis.UnitTests -> Microsoft.NET.Test.Sdk 17.12.0 -> Microsoft.TestPlatform.TestHost 17.12.0 -> Microsoft.TestPlatform.ObjectModel (>= 17.12.0)  [C:\git\dotnet\src\roslyn\Roslyn.sln]
C:\git\dotnet\src\roslyn\src\Compilers\Core\CodeAnalysisTest\Microsoft.CodeAnalysis.UnitTests.csproj : error NU1109:  Microsoft.CodeAnalysis.UnitTests -> Microsoft.TestPlatform.ObjectModel (>= 17.5.0) [C:\git\dotnet\src\roslyn\Roslyn.sln]

The fix is to explicitly set the version of Microsoft.NET.Test.Sdk that is consumed by arcade version that Roslyn uses.

[jjonescz]

_MicrosoftTestPlatformVersion property seems to be unused now, should it be removed?

[NikolaMilosavljevic]

Oh yeah, missed that - I'll send the update.

[NikolaMilosavljevic]

Fixed with f252374