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.
@mainakjas FYI I changed these tests to use global variables to allow the next parts of the code to execute. This means less forced waiting locally, and at the same time *should* reduce how often Travis hits a timeout problem.