clients/tests: increase timeout for asynchronous jobs

The default timeout is really only here to abort the test when it
definitely failed to avoid keeping it hanging forever. It's not part
of any strict assertions and should be large.

Fixes: bb4b749595 ('clients/tests: don't wait for first job before scheduling parallel jobs')

clients/tests/test-client.py

@@ -444,7 +444,7 @@ class AsyncProcess():
                  args,
                  env,
                  complete_cb,
-                 max_waittime_msec = 3000):
+                 max_waittime_msec = 20000):
         self._args = list(args)
         self._env = env
         self._complete_cb = complete_cb