github: Use "check-runtimes" rather than individual "check-cxx" etc

This makes sure to run all three testsuites, even if there are
failures in one of them.

.github/workflows/test-libcxx.yml

@@ -102,4 +102,4 @@ jobs:
             -DCMAKE_INSTALL_MESSAGE=NEVER `
             -DLLVM_LIT_ARGS="-v --time-tests"
           ninja
-          ninja check-cxx check-cxxabi check-unwind
+          ninja check-runtimes