Improve SourceGeneratorTests test speed.

[chsienki]

Summary

Cache the fully-constructed base Project in source generator tests so it is built once and shared across all ~85 test invocations, reducing test execution time by ~60%.

Problem

CreateBaseProject() was called per-test, each time independently:

• Loading DependencyContext via reflection
• Scanning 200+ DLLs via Directory.EnumerateFiles
• Creating MetadataReference.CreateFromFile() for each DLL
• Performing O(n²) dedup via .Any() on all existing references
• Adding references one-at-a-time, creating new immutable Solution snapshots per add

The resulting reference set was identical every time.

Fix

• Cache the entire base Project (workspace, compilation options, parse options, and all metadata references) in a static Lazy<Project>. Since Project is immutable, every AddDocument/AddAdditionalDocument call in individual tests returns a new fork without affecting the shared instance.
• The 2 tests that need custom CSharpParseOptions fork via WithParseOptions().
• Replace the O(n²) dedup loop (.Any() per DLL) with Dictionary.TryAdd for O(1) lookups during the one-time initialization.

Results

Metric	Before	After
Total time (incl. build)	~51s	~20s