test: do not leave process hanging in test-namespace

It is killed when the main test process exists, but still,
it will be left hanging while other test cases run, so it's
not very clean.

Follow-up for 8b5e3be88e

src/test/test-namespace.c

@@ -425,7 +425,7 @@ TEST(namespace_get_leader) {
 }
 
 TEST(detach_mount_namespace_harder) {
-        _cleanup_(pidref_done) PidRef pid = PIDREF_NULL;
+        _cleanup_(pidref_done_sigkill_wait) PidRef pid = PIDREF_NULL;
         _cleanup_close_pair_ int p[2] = EBADF_PAIR;
         char x = 0;
         int r;