Build one test project per Roslyn version

[meziantou]

The analyzer and the code fixers already had one project per Roslyn version, but the tests did not: a single test project was built against whatever /p:RoslynVersion was passed to it. This applies the same pattern to the tests, and removes the duplication that made it necessary.

Tests follow the src pattern

One Meziantou.Analyzer.Test.roslyn<version>.csproj per version, with the shared content moved to the Directory.Build.props of the folder. Each test project references the analyzer and the code fixers of its own Roslyn version.

The development projects are gone

Meziantou.Analyzer.csproj, Meziantou.Analyzer.CodeFixers.csproj and Meziantou.Analyzer.Test.csproj are removed, as every project is now tied to a Roslyn version. DocumentationGenerator is not specific to a version, so it references Meziantou.Analyzer.$(RoslynVersion).csproj and follows the default. Its Version="4.11.0" on Microsoft.CodeAnalysis.CSharp.Workspaces was already dead (the Update in Directory.Build.targets overrode it to 5.9.0), so it was dropped.

/p:RoslynVersion is no longer a way to build another version — build or test the project of that version instead. The version specific projects keep TreatAsLocalProperty="RoslynVersion" so a stray /p:RoslynVersion cannot silently retarget them.

Less to maintain per version

RoslynVersion is derived from the name of the project file in the Directory.Build.props of the repository root, so the 15 version specific projects are now empty. The version of the Microsoft.CodeAnalysis.* packages is derived from it too, replacing the 15 hardcoded entries in Directory.Build.targets; what stays version specific there is DefineConstants and the warnings to disable.

Removing the Otherwise branch created a silent failure path: a project named after an unsupported version would have compiled with no ROSLYN_* constant at all, quietly dropping #if blocks while still succeeding. Each When now sets RoslynVersionSupported and a BeforeBuild target errors otherwise.

CI

The build_and_test matrix is discovered from the test projects present in the repository instead of being hardcoded, and a list_test_projects job validates that every Roslyn version has an analyzer, a code fixer and a test project, and that the default version is the latest supported one. The version comparison casts to [version], so roslyn5.10 sorts after roslyn5.9.

Also removed runs-on and configuration from the matrix, as each had a single value.

Note for reviewers

All the test projects produce an assembly named Meziantou.Analyzer.Test (required by the InternalsVisibleTo in AssemblyInfo.cs), so on a solution wide run they all wrote the same trx file and five reports were silently overwritten. Each project now sets its own report name. CI was never affected, as it runs one project per job.

build/update-version.ps1 referenced the deleted Meziantou.Analyzer.csproj, but it was already dead before this change: it targets a PackageVersion property and a src/Meziantou.Analyzer.Vsix/ folder that no longer exist, and the CI computes the version itself. Left untouched, but it looks like a candidate for deletion.

Verification

• Solution builds with 0 warnings and 0 errors.
• 17,921 tests pass across the 5 Roslyn versions, and each project writes its own trx file.
• Resolved package versions checked project by project against the removed table: identical.
• dotnet run --project src/DocumentationGenerator exits 0 with byte identical markdown.
• The packed nupkg contains the analyzers for the 5 versions and nothing under lib/.
• Both CI PowerShell steps were run against the real tree, including the failure paths: missing test project, orphan test project, missing code fixer, a newer version added without bumping the default, and a default pointing at a removed version.