FIX: Cleaner n2ft test

Had a random failure on the new `neuromag2ft` test, I *think* this will help the problem. Going to restart the tests a few times to see.
oops, late to review. I don't know how global variables work with threads but I have been thinking that the best way to get rid of these errors is to use `Queue.Queue` instead of a `time.sleep` (which varies from machine to machine) so that we can get tight control over the timings. That's how it works in `test_fieldtrip_client.py`