qa: Intermittent error in wallet_resendwallettransactions.py

[hebasto]

https://travis-ci.org/github/hebasto/bitcoin/jobs/735078189#L5109:

AssertionError: Predicate ''''
        def test_function():
            if check_connected:
                assert self.is_connected
            return test_function_in()
''' not true after 240.0 seconds

[maflcko]

The scheduler thread runs in a different thread than the RPC thread and message handling thread. As the mockscheduler call is non-blocking and there is no way to observe to-be-trickled txids over RPC or P2P, the only way to synchronize this event I see is over assert_debug_log. Fixed in #20112

Longer term, mockscheduler should probably be blocking until all events that are scheduled in the past have been executed.