Add extra test assertions around tests

[Youssef1313]

No description provided.

[Copilot]

Pull request overview

This PR tightens test-run invariants in the shared xUnit test framework wrapper to detect leakage of global MSBuild test state across test execution, with an opt-out for OM unit tests.

Changes:

• Add BuildEnvironmentState.s_runningTests assertions around PreInvoke, PostInvoke, and the test-case Run lifecycle.
• Capture and assert stability of selected Microsoft.Build.Framework.Traits values across each test case run (excluded for OM tests).
• Define IS_OM_TESTS in the OM unit test project to compile out the Traits assertions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Shared/UnitTests/TestAssemblyInfo.cs	Adds invariant assertions around xUnit invocation and test execution; introduces #if !IS_OM_TESTS guard for Traits stability checks.
src/Build.OM.UnitTests/Microsoft.Build.Engine.OM.UnitTests.csproj	Defines IS_OM_TESTS compilation constant to exclude the Traits assertions for OM tests.

[Youssef1313]

@rainersigwald @JanProvaznik This only makes assertions stronger, and simplifies a test I recently modified with the xunit.v3 update and fixes the comment I added that seems to be partially wrong or outdated.