tests/client: increase timeout to wait for process

We now get unit test failures hitting this timeout. That is
likely a new bug introduced somewhere, but to rule out that
the timeout is simply too short, increase it.

src/tests/client/test-client.py

@@ -575,7 +575,7 @@ class NMStubServer:
 
 
 class AsyncProcess:
-    def __init__(self, args, env, complete_cb, max_waittime_msec=20000):
+    def __init__(self, args, env, complete_cb, max_waittime_msec=40000):
         self._args = list(args)
         self._env = env
         self._complete_cb = complete_cb