enable negative assertions on output

[shadeofblue]

the problem:

we'd like to ensure something does not appear in the output of the executed command

if we use:

            with pytest.raises(AssertionError):
                await cmd_monitor.wait_for_pattern(".*Task finished by provider", timeout=30)

this completes successfully but any subsequent assertions on output will immediately exit with an asyncio.CancelledError because of the changed state (of the internal PatternMatchingEventMonitor object ?)...

it should be possible to assert negatively on output within a given timeout without affecting results of other assertions...