[5.0] Let consumers choose the Roslyn version used by the testing framework

[josefpihrt]

Problem

The Roslynator testing framework forced every test run onto a fixed Roslyn version (4.14.0), because Roslynator.Testing.* declared Microsoft.CodeAnalysis.* >= 4.14.0. Since NuGet dependency versions are minimums, a consumer whose analyzer targets an older Roslyn (e.g. the roslyn3.8 / roslyn4.7 buckets Roslynator itself ships) could not test against that version — NuGet unified the graph up to 4.14.0.

Change

• Lower the testing packages' Roslyn floor to 3.8.0 so the effective version is determined by the consumer's own Microsoft.CodeAnalysis.* reference. Verified the Testing.* projects use no API newer than 3.8. build_core_and_testing compiles and packs those projects against 3.8.
• Decouple Roslynator.Testing.Common from the Roslynator.Core package (its floor would otherwise leak back through). The ~6 internal helpers used (StringBuilderCache, Hash, the diagnostic comparers, ToReportDiagnostic) are duplicated into Testing.Common as internal types. Roslynator.Core's own published floor is unchanged.
• Add RoslynatorTestRoslynVersion (default 5.0.0, matching this repo's analyzer Microsoft.CodeAnalysis.* floor after Bump Roslyn to 5.0 #1787) so the in-repo suite pins the Roslyn version used at test runtime.
  • This property only pins the test projects' Microsoft.CodeAnalysis.* PackageReference. It does not switch RoslynatorRoslynVersion / analyzer compilation.
  • This repo cannot pin the suite below 5.0: test projects ProjectReference analyzers that require Microsoft.CodeAnalysis.* >= 5.0.0.
  • CI does not run the analyzer suite at 3.8.0: that is the testing package compile floor for consumers, not a version this repo's analyzers can execute against.
• Guard version-sensitive tests (SyntaxKindTests and tests using C# 12/13 features or newer-Roslyn formatter output) with ROSLYN_TEST_* defines in src/Tests/Directory.Build.props.
• Also fix the roslyn4.7 DefineConstants list: ROSLYN_4_2,ROSLYN_4_4 used a comma, so ROSLYN_4_4 was never defined for that bucket. It is now semicolon-separated (already on main via Bump Roslyn to 5.0 #1787).

Consumer action required

A test project that relied on the implicit 4.14.0 should now add its own reference:

<PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="4.14.0" />

The chosen version raises the maximum C# the parser can accept; test sources still parse at CSharpParseOptions.Default (not Latest) unless you set parse options / LanguageVersion. The same note is in the Testing.CSharp.Xunit / MSTest NuGet readmes.

A test project that used Roslynator.Core types via the old transitive dependency must add an explicit Roslynator.Core package reference.

Verification

• Full suite green at RoslynatorTestRoslynVersion=5.0.0 (default).
• Packed nuspecs confirm Testing.Common/Testing.CSharp floor at 3.8.0 with no Roslynator.Core dependency; Roslynator.Core still floors at the product Roslyn version.

[josefpihrt]

Review notes for making the testing framework’s Roslyn version consumer-selectable. Direction and changelog/breaking-change notes look strong; a few verification/CI clarifications.

[josefpihrt]

Detailed pass on the testing-framework Roslyn floor change. The approach is right: compile Testing.* against 3.8, drop the Roslynator.Core leak, pin this repo with RoslynatorTestRoslynVersion, and document the consumer PackageReference. CI is green at default 4.14 and at test_roslyn_versions 4.7.0; the earlier review nits on the 4.4 DefineConstants comma and the CI comment are addressed.

A few leftover product/docs issues before merge.