core: increase timeout for test_nm_utils_kill_child()

The test starts a watchdog process that eventually kills the process
group after a certain time. When running under valgrind concurrently
to other tests the current timeout is not enough; increase it.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/2067
(cherry picked from commit 23ffa5fc6e)
(cherry picked from commit e17f1b93ec)

src/core/tests/test-core-with-expect.c

@@ -229,7 +229,7 @@ do_test_nm_utils_kill_child(void)
     char          *argv_watchdog[] = {
         "bash",
         "-c",
-        "sleep 4; "
+        "sleep 15; "
                  "kill -KILL 0; #watchdog for #" TEST_TOKEN,
         NULL,
     };