Migrate the tests to the Roslyn SDK testing harness and remove ProjectBuilder

[meziantou]

Migrates the whole test suite from the hand-written ProjectBuilder harness to the Roslyn SDK testing packages (Microsoft.CodeAnalysis.CSharp.Analyzer.Testing and siblings, already referenced), and deletes the old harness.

What changed

All 194 test classes migrated — 189 rule classes, 3 suppressor classes, 2 refactoring classes. Tests now follow one shape:

[Fact]
public Task SomeCase()
{
    var test = CreateTest();
    test.TestCode = """...{|MA0001:code|}...""";
    test.FixedCode = """...""";

    return test.RunAsync();
}

Old harness removed: ProjectBuilder, ProjectBuilder.Validation, its own validation tests, DiagnosticResult, DiagnosticResultLocation, TargetFramework, TestAnalyzerConfigOptionsProvider and AnalyzerAssemblyLoader, plus the TestHelper global using. Helpers/ keeps only GeneratedCodeAnalysisAnalyzer and SharedHttpClient, which the new harness still uses.

New harness under tests/Meziantou.Analyzer.Test/Harness/, holding the repository defaults (net11.0 reference assemblies, LanguageVersion.Latest, MetadataImportOptions.All) and the features the migration needed:

• CSharpAnalyzerTest, CSharpCodeFixTest, CSharpCodeRefactoringTest with a parameterless RunAsync() that supplies the test's cancellation token
• AdditionalAnalyzers — run several analyzers in one test
• UseFrameworkSourceGenerators — runs the real source generators from the .NET reference pack the test compiles against, so [GeneratedRegex] partial members need no hand-written implementation. GeneratorAssemblyLoader isolates each pack in its own AssemblyLoadContext, since different packs ship same-named assemblies at different versions
• GeneratedCodeAnalysisFlags, ProjectState.SetConfiguration(...), AddMeziantouAnnotations(), AddAspNetCore()/AddMSTestApi()/AddXUnitApi()/AddNUnitApi()/AddSystemTextJson(), and AddMicrosoftCodeAnalysisNetAnalyzers()/AddMicrosoftCodeAnalysisCSharpCodeStyleAnalyzers() for the external analyzers the suppressor tests need
• NuGetPackages — the package download and cache extracted from ProjectBuilder

Markup is always explicit. Every diagnostic uses {|MA0001:code|}; the bare [|code|] form is gone from the analyzer tests (1084 markers rewritten). The [|…|] in Refactorings stays — there it is the selection span, not a diagnostic.

Production fixes

The new harness compiles fixed code and validates syntax trees, which the old one did not. That surfaced real defects, all fixed here:

• 21 code fixers built A.B.C as a QualifiedName where an expression node was required, producing a tree that does not round-trip through the parser. They now use a new SyntaxGeneratorExtensions.TypeMemberAccessExpression helper. (One spot in UseStringEqualsFixer deliberately keeps the qualified form: the Simplifier relies on it to reduce a static call to an extension method.)
• SimplifyNegatedBooleanExpressionFixer rebuilt the node with the original operator kind instead of the new one.
• OptimizeGuidCreationFixer emitted literal tokens whose text and value disagreed.
• UseRegexSourceGeneratorFixer checked the generated member's name only against type members, so a name taken from a local collided with it and the call bound to the local (CS0149/CS0165). It now also requires the name to be free in the scope it is used from.
• UseRegexSourceGeneratorAnalyzer reported MA0110 for a Regex whose match timeout [GeneratedRegex] cannot express (TimeSpan.Zero, negative values other than Infinite); applying the fix produced code the generator rejects. The rule is no longer reported for those, documented in docs/Rules/MA0110.md.

Notes for the reviewer

• One test was removed: AvoidUsingRedundantElseAnalyzerTests.Test_LongElseIfChainWhereEveryBranchJumps_AllElsesReported. Its 1000-deep else if chain overflows the testing library's analyzer driver and kills the test host, silently truncating whole runs. It was a performance guard against quadratic re-analysis; a shorter chain would restore that coverage without overflowing.
• A few tests declare compiler diagnostics on purpose and should stay: SYSLIB1044 in UseRegexTimeoutAnalyzerTests (the real generator cannot fully implement a NonBacktracking pattern) and CS0841/CS0165 in EventsShouldHaveProperArgumentsAnalyzerTests (deliberately invalid inputs).
• 13 _PartialProperty tests in UseRegexSourceGeneratorAnalyzerTests moved from .NET 7 to .NET 9 reference assemblies: [GeneratedRegex] is only valid on a property from .NET 9, and their fixed code was previously never compiled.
• CA1507_STJ_JsonPropertyName is vacuous — with NetAnalyzers 10.0.100, CA1507 is not reported on [JsonPropertyName] at all, so nothing exercises that branch of the suppressor. Left as a guard against the rule reporting again; worth deciding whether the branch still earns its keep.

Verification

Clean dotnet build and the full suite pass on every supported Roslyn version, with no build overrides:

Version	Tests	Failed
roslyn4.8	3740	0
roslyn4.14	3767	0
roslyn5.0	3795	0
roslyn5.6	3819	0
roslyn5.9	3852	0

dotnet run --project src/DocumentationGenerator exits 0. Test counts were checked class by class against the merge base throughout, so no assertions were dropped in the conversion.