Fix test target buildability with test-case-level skips

Selective testing and sharded builds now keep a test target buildable when only individual test cases are skipped. Previously, a test-case-level skip such as Target/Class/method could cause the entire target to be dropped from the scheme, leading to “The scheme cannot be built because it contains no buildable targets” or a falsely successful run that never rebuilt the target. Now only whole-target skips remove the target from the build.