Module Cache support for test helper frameworks

Module Cache can now cache framework targets that support your tests, even when they link XCTest or Swift Testing. This helps projects with shared assertion helpers, fixtures, or test utilities reuse binary cache artifacts instead of rebuilding those helper modules from source every time.

Test bundles themselves are still left out of the binary cache. The change only applies to regular framework products that tests depend on, so your test targets keep their normal behavior while their reusable support modules can benefit from local and remote cache hits.