Replace StubFilesExtensionLoaderTest with ResultCacheNoReflectionTest

Test that BetterReflection is not initialized when ResultCacheManager
constructs the ResultCache, which is the real-world code path that
exercises StubFilesExtensionLoader::getFiles().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: xificurk