Mixed app-hosted and hostless test bundles no longer crash the test runner

When the autogenerated workspace scheme mixes app-hosted and hostless test bundles, default tuist test runs no longer crash the test runner during bootstrap. The CLI now runs the underlying generated project schemes separately instead of the mixed workspace aggregate. --test-targets and --skip-test-targets apply per split scheme so requesting FeatureTests does not also run AppTests. Explicit --scheme selection is unchanged.