Keep selective test runs buildable when a test case is skipped

When using tuist test with selective testing, a test target that is the only remaining target to run is now kept buildable even when a single test case in it is skipped. Previously, a test-case-level skip such as Target/Class/method was treated as a whole-target skip, causing the error “The scheme cannot be built because it contains no buildable targets”. The skipped test case is still passed to xcodebuild individually.