test: Don't allocate scope if already running in unit with delegated cgroup

If we're already running in a unit with delegation turned on, let's
skip allocation of a scope unit and cgroup subroot. This allows journald
to correctly attribute the logs of all subprocesses spawned by tests such
as test-execute to the test-execute service when the test is running in a service.

src/basic/log.c

@@ -256,7 +256,7 @@ fail:
         return r;
 }
 
-static bool stderr_is_journal(void) {
+bool stderr_is_journal(void) {
         _cleanup_free_ char *w = NULL;
         const char *e;
         uint64_t dev, ino;