tests: Skip test-cgroup-util test_mask_supported() when not running under systemd

Commit 5f4c5fef6 introduced this new test case, but this does not work in
build chroots where cgroupfs is not mounted. So skip the test if systemd is not
running.

src/test/test-cgroup-util.c

@@ -320,7 +320,7 @@ int main(void) {
         test_controller_is_valid();
         test_slice_to_path();
         test_shift_path();
-        test_mask_supported();
+        TEST_REQ_RUNNING_SYSTEMD(test_mask_supported());
 
         return 0;
 }