Add support for testing IIncrementalGenerator

[mhutch]

Draft for now as I'm not certain this is the right approach or whether it's okay to increase the version of the Roslyn dependency.

Fixes #941.

[AraHaan]

No need to make new types (I think), one can add , IIncrementalGenerator, new() to the contraints on the ones already accepting an ISourceGenerator that I see so far.

Let me go to definition dive for all that needs changed.

[AraHaan]

public class CSharpSourceGeneratorTest<TSourceGenerator, TVerifier> : SourceGeneratorTest<TVerifier>
-         where TSourceGenerator : ISourceGenerator, new()
+         where TSourceGenerator : ISourceGenerator, IIncrementalGenerator, new()
        where TVerifier : IVerifier, new()

In SourceGeneratorTest that is inherited from this change:

• GetSourceGenerators to return object instead of ISourceGenerator so that way CSharpSourceGeneratorTest would just work with it (have it down cast to object) then have SourceGeneratorTest do a type check then cast operation to ISourceGenerator or to IIncrementalGenerator.
• CreateGeneratorDriver to be defined as protected abstract GeneratorDriver CreateGeneratorDriver(Project project, ImmutableArray<object> sourceGenerators); (again when using it would need to type check and cast to use the instances).
• ApplySourceGeneratorAsync to be defined as private async Task<(Project project, ImmutableArray<Diagnostic> diagnostics)> ApplySourceGeneratorAsync(ImmutableArray<object> sourceGenerators, Project project, IVerifier verifier, CancellationToken cancellationToken)
• VerifySourceGeneratorAsync to be defined as private async Task<ImmutableArray<Diagnostic>> VerifySourceGeneratorAsync(string language, ImmutableArray<object> sourceGenerators, SolutionState testState, Func<ImmutableArray<object>, Project, IVerifier, CancellationToken, Task<(Project project, ImmutableArray<Diagnostic> diagnostics)>> getFixedProject, IVerifier verifier, CancellationToken cancellationToken)