Fix flaky test: eliminate parallel build conflict in TestDiscoveryWarningsTests

[Copilot]

TestDiscoveryWarningsTests was intermittently failing with file-locking errors (IOException: The process cannot access the file ... because it is being used by another process) because TestAssetFixtureBase.InitializeAsync builds all assets in parallel, and TestDiscoveryWarnings has a ProjectReference to TestDiscoveryWarningsBaseClass — so both parallel builds raced to write the same output files.

Changes

• Embed TestDiscoveryWarningsBaseClass as a subdirectory within the TestDiscoveryWarnings asset instead of yielding it as a separate asset from GetAssetsToGenerate. Only one explicit build is now triggered; MSBuild handles the base class as a project reference dependency.
• Update ProjectReference from ../TestDiscoveryWarningsBaseClass/... to TestDiscoveryWarningsBaseClass/... to reflect the new relative path.
• Remove the now-unnecessary BaseTargetAssetPath property and BaseClassSourceCode constant.

Before (two parallel builds → race condition):

yield return (BaseClassAssetName, BaseClassAssetName, BaseClassSourceCode...);
yield return (AssetName, AssetName, SourceCode...);  // ProjectReference: ../TestDiscoveryWarningsBaseClass/...

After (single build, base class as subdirectory):

yield return (AssetName, AssetName, SourceCode...);  // ProjectReference: TestDiscoveryWarningsBaseClass/...
// BaseClass files live at #file TestDiscoveryWarningsBaseClass/TestDiscoveryWarningsBaseClass.csproj

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[Evangelink]

@copilot tests are failing:

test/Utilities/Microsoft.Testing.TestInfrastructure/DotnetCli.cs(176,1): error : [DiscoverTests_ShowsWarningsForTestsThatFailedToDiscover ("net10.0")] [net11.0] Class Initialization method MSTest.Acceptance.IntegrationTests.TestDiscoveryWarningsTests.ClassInitialize threw exception. System.InvalidOperationException: Command 'dotnet build /mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/kl3sC/TestDiscoveryWarnings -c Release -p:MSBuildTreatWarningsAsErrors=true -p:TreatWarningsAsErrors=true -p:SuppressNETCoreSdkPreviewMessage=true -bl:"/mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/TestDiscoveryWarnings-119.binlog"' failed.

StandardOutput:
Telemetry is: Disabled
  Determining projects to restore...
  Restored /mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/kl3sC/TestDiscoveryWarnings/TestDiscoveryWarningsBaseClass/TestDiscoveryWarningsBaseClass.csproj (in 605 ms).
  Restored /mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/kl3sC/TestDiscoveryWarnings/TestDiscoveryWarnings.csproj (in 749 ms).
  TestDiscoveryWarningsBaseClass -> /mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/kl3sC/TestDiscoveryWarnings/TestDiscoveryWarningsBaseClass/bin/Release/net10.0/TestDiscoveryWarningsBaseClass.dll
/mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/kl3sC/TestDiscoveryWarnings/UnitTest1.cs(8,27): error CS0436: The type 'BaseClass' in '/mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/kl3sC/TestDiscoveryWarnings/TestDiscoveryWarningsBaseClass/UnitTest1.cs' conflicts with the imported type 'BaseClass' in 'TestDiscoveryWarningsBaseClass, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'. Using the type defined in '/mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/kl3sC/TestDiscoveryWarnings/TestDiscoveryWarningsBaseClass/UnitTest1.cs'. [/mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/kl3sC/TestDiscoveryWarnings/TestDiscoveryWarnings.csproj::TargetFramework=net10.0]
Build FAILED.
/mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/kl3sC/TestDiscoveryWarnings/UnitTest1.cs(8,27): error CS0436: The type 'BaseClass' in '/mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/kl3sC/TestDiscoveryWarnings/TestDiscoveryWarningsBaseClass/UnitTest1.cs' conflicts with the imported type 'BaseClass' in 'TestDiscoveryWarningsBaseClass, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'. Using the type defined in '/mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/kl3sC/TestDiscoveryWarnings/TestDiscoveryWarningsBaseClass/UnitTest1.cs'. [/mnt/vss/_work/1/s/artifacts/tmp/Debug/testsuite/kl3sC/TestDiscoveryWarnings/TestDiscoveryWarnings.csproj::TargetFramework=net10.0]
    0 Warning(s)
    1 Error(s)
Time Elapsed 00:00:07.39
StandardError:
.

[Copilot]

@copilot tests are failing:

...

Fixed in 18b9e19. The issue was that MSBuild's default **/*.cs glob in the parent project was picking up TestDiscoveryWarningsBaseClass/UnitTest1.cs, causing the type conflict. Added <Compile Remove="TestDiscoveryWarningsBaseClass/**" /> to exclude the subdirectory's source files from the parent project's compilation.

[Copilot]

Pull request overview

Fixes intermittent TestDiscoveryWarningsTests failures caused by TestAssetFixtureBase.InitializeAsync building multiple generated assets in parallel while one asset had a ProjectReference to another, leading to concurrent MSBuild writes (file locking).

Changes:

• Stops generating TestDiscoveryWarningsBaseClass as a separate asset; instead embeds it as a subdirectory within the TestDiscoveryWarnings asset so only one explicit build runs.
• Updates the ProjectReference path to the new in-asset location.
• Removes the now-unused BaseTargetAssetPath and BaseClassSourceCode constant by folding base-class files into the main SourceCode asset string.

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[Copilot]

<Compile Remove="TestDiscoveryWarningsBaseClass/**" /> uses a very broad glob that will enumerate all files under that subtree (including bin/obj once they exist) during MSBuild evaluation. Since the intent is only to prevent the main project from compiling the nested base-class sources, restrict the remove pattern to C# sources (e.g., **/*.cs) to avoid unnecessary filesystem scanning and accidental future removals if default items change.

Suggested change

	<Compile Remove="TestDiscoveryWarningsBaseClass/**" />
	<Compile Remove="TestDiscoveryWarningsBaseClass/**/*.cs" />