tests: ninja is not ninja on RHEL7/ppc64/Epel, but cmake knows

By using `cmake --build` instead of calling `ninja` directly, test
should run well on ppc64 even when `ninja-build` is all we have.

tests/integration-test-suite/test.sh

@@ -34,4 +34,4 @@ git clone $repo_url
 cd llvm-toolchain-integration-test-suite
 mkdir _build && cd _build
 cmake .. -GNinja ${CMAKE_CXXLIB:-}
-ninja $thread_args check
+cmake --build . $thread_args --target check