Convert System.Collections.Tests to xUnit v3

[agocke]

Summary

• add a per-project UseXUnitV3 opt-in to the libraries test infrastructure
• build a separate xUnit v3 variant of TestUtilities so v2 and v3 projects can coexist
• convert the standard desktop/archive System.Collections.Tests path to xUnit v3
• keep single-file, NativeAOT, ReadyToRun, mobile, browser, and WASI configurations on xUnit v2 for this pilot

Validation

• dotnet build src/libraries/System.Collections/tests/System.Collections.Tests.csproj -c Release
• xUnit v3 smoke run of System.Collections.Tests.StructuralComparisonsTests: 21 passed
• dotnet build src/libraries/System.Collections.Concurrent/tests/System.Collections.Concurrent.Tests.csproj -c Release to verify a neighboring xUnit v2 project remains unaffected

A full local /t:test run is currently blocked by the existing testhost layout: the Release testhost contains .NET 11 while the generated runtime configuration requires .NET 12. Refreshing the prescribed baseline is blocked because clang-18 is not installed on this machine.

Note

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