doh_test: change slowdown logic so that it will not stuck

There is a slight chance that active_cconnects could exceed the number
of workers, making the unit test stuck.

tests/isc/doh_test.c

@@ -725,6 +725,7 @@ doh_connect_thread(void *arg) {
 	 */
 	int_fast64_t active = atomic_fetch_add(&active_cconnects, 1);
 	if (active > workers) {
+		(void)atomic_fetch_sub(&active_cconnects, 1);
 		goto next;
 	}
 	connect_send_request(connect_nm, req_url, atomic_load(&POST),