test-bpf-devices: skip test on !unified

The code in cgroup.c has support for all hierarchies, but the test,
as written, will only work on unified. Since the test is really about
bpf code, and not the legacy devices controller, let's just skip
the test.

src/test/test-bpf-devices.c

@@ -263,6 +263,10 @@ int main(int argc, char *argv[]) {
         rl.rlim_cur = rl.rlim_max = MAX(rl.rlim_max, CAN_MEMLOCK_SIZE);
         (void) setrlimit(RLIMIT_MEMLOCK, &rl);
 
+        r = cg_all_unified();
+        if (r <= 0)
+                return log_tests_skipped("We don't seem to be running with unified cgroup hierarchy");
+
         if (!can_memlock())
                 return log_tests_skipped("Can't use mlock()");