Remove redundant .NET and language version settings in tests

[meziantou]

What

Removes the explicit target framework, ASP.NET Core and C# language version settings in the tests when the defaults of the testing harness are enough.

• ~200 test.ReferenceAssemblies = ReferenceAssemblies.Net.NetXX; assignments removed, plus the equivalent object initializers (new CodeFixTest { ReferenceAssemblies = ... }).
• Pinned ASP.NET Core and NuGet combinations replaced by the shared defaults: ReferenceAssemblies.Net.Net80.AddAspNetCore("8.0.0") becomes test.ReferenceAssemblies.AddAspNetCore(), and ...Net80.AddPackages([...]) becomes test.ReferenceAssemblies.AddPackages([...]).
• 188 explicit test.LanguageVersion = ... assignments removed, plus 13 more in UseStringComparerAnalyzerTests that only existed to lift the C# 9 class default back up to C# 12.
• UseRegexSourceGeneratorAnalyzerTests gets a CreatePartialMethodTest helper (.NET 7 + C# 11) instead of repeating the language version in 23 tests, which also lets the partial property tests drop their Net90 overrides.
• The now unused using Microsoft.CodeAnalysis.Testing; and using Microsoft.CodeAnalysis.CSharp; directives are removed.

Why

Most tests were pinning a framework or a language version that predates the default of the harness, which hides which tests actually depend on a specific version.

What is kept

Every setting that is load bearing was verified by removing it and observing the failures:

• Tests that assert version specific behavior (ProcessWaitForExit_NET5, Net5_NoDiagnostic, RefStruct_CSharp12, CannotBeReadOnly_CSharp7, InjectProperty_AspNetCore8_NoDiagnostic, ...).
• LanguageVersion.Preview for union types, closed types and System.Threading.Lock: Latest is below those features on the older Roslyn versions.
• The regex source generator tests: the .NET 11 generator emits collection expressions, which need C# 12, and the code fixer generates a partial property as soon as the language supports it.
• The netstandard and net48 targets, and the DI based tests whose types are not in the .NET 11 ASP.NET Core reference pack.

Testing

dotnet test --max-parallel-test-modules 2: 19013 passed, 0 failed, on Roslyn 4.8, 4.14, 5.0, 5.6 and 5.9.

Note for the reviewer

Names such as Dictionary_String_CollectionExpression_DefaultOnCSharp12_ShouldNotReportDiagnostic now run with the default language version instead of exactly C# 12. The behavior is identical, but tell me if you prefer to keep the explicit pin so the names keep matching.