Clean up the test harness

[meziantou]

What changed

Analyzer

• MA0060 (DoNotIgnoreReturnValueAnalyzer) now declares a single descriptor. ReturnValueRule and OutParameterRule shared the MA0060 id, which forced every test of that rule to use MarkupOptions.UseFirstDescriptor. They are merged into one Rule with messageFormat: "{0}" (the pattern already used by OptimizeStringBuilderUsageAnalyzer and others), and the message is composed at the report site. The reported messages are unchanged, suffix included:
  • The return value of 'X' should be used (+ : <Message> from [DoNotIgnore])
  • The out parameter 'x' of 'M' should not be discarded (+ : <Message>)

Tests

• Removed the redundant CodeFixTestBehaviors.FixOne | CodeFixTestBehaviors.SkipFixAllCheck pairs. The two flags are orthogonal and the pairing is valid, but only 7 of the 13 sites needed a flag at all: 4 tests in AvoidUnusedInternalTypesAnalyzerTests keep SkipFixAllCheck (the fix removes the file's only type, so the harness cannot compare a state with no document) and 3 in OptimizeStringBuilderUsageAnalyzerTests keep FixOne (the fix reveals another diagnostic). Each removal was verified by re-running the affected classes.
• DoNotIgnoreReturnValueAnalyzerTests drops UseFirstDescriptor and, now that the message is assertable, pins both message shapes with {|#0:…|} + WithMessage(...).
• All package references go through named extension methods on ReferenceAssemblies with an optional version — AddSerilog(), AddSqlite(), AddEntityFrameworkCore(), AddNewtonsoftJson(), AddMoq(), … — instead of inline AddPackages([new PackageIdentity(...)]). Default versions are the latest stable ones (MSTest 4.3.3, NUnit 4.6.1, xUnit v3 4.0.0, EF Core 10.0.11, Serilog 4.4.0, YamlDotNet 18.1.0, …); a test that needs a specific version passes it.
• Renamed AddXUnitApi → AddXunit (now xUnit v3), AddMSTestApi → AddMSTest, AddNUnitApi → AddNUnit. Added AddXunitV2 for the test that asserts the pre-Xunit.TestContext behavior. Removed AddSystemTextJson and AddSystemCollectionsImmutable, which only existed because a few tests compiled against an older target framework.
• DoNotUseBlockingCallInAsyncContextAnalyzer_AsyncContextTests no longer pins netstandard2.1 for the whole class (it dated back to needing ValueTask in 2020). Only ProcessWaitForExit_NET5 depended on it and now pins Net50 explicitly, matching its name and its ProcessWaitForExit_NET6 counterpart.
• The 6 UseStringComparerAnalyzerTests immutable-collection tests no longer target net48 + a System.Collections.Immutable package; nothing in them is framework specific.

Build / CI

• paths-ignore of the push trigger now includes tests/**, so a push to main touching only tests does not run the workflow and does not publish a package. Pull requests still run the full matrix.
• Bumped Meziantou.NET.Sdk, .Test and .Web to 1.0.161.
• Documented why Microsoft.Bcl.AsyncInterfaces must stay: the suppressor tests Assembly.LoadFrom the IDE code-style analyzers, which need it in the output folder.

Notes for reviewers

• Two test snippets had to be adapted to the newer test frameworks: MSTest 4 cannot infer T for AreEqual(null, true), and NUnit 4 moved Assert.AreEqual to ClassicAssert (which MA0003 does not exempt), so the snippet is now Assert.That(true, Is.True) — still a boolean literal on NUnit.Framework.Assert, so it exercises the same exemption. Assert.That(null, Is.Null) was rejected because it is ambiguous under Roslyn 4.8.
• DbContext_Add moved off Net60 because EF Core 10 does not support net6.0.
• global.json was updated despite the AGENTS.md rule, at the maintainer's request.

Validation

Full test suite passes on every supported Roslyn version: 4.8 (3748), 4.14 (3775), 5.0 (3803), 5.6 (3827), 5.9 (3860) — 0 failures. dotnet build is clean and dotnet run --project src/DocumentationGenerator reports no documentation change.