Test suite hangs when both wxPython and PySide2 installed

[mdickinson]

In an environment with both wxPython and PySide2 installed, the test suite hangs. It seems likely that there's some interaction between the GUI setups for the two frameworks.

Not really high priority (the obvious solution is "don't do that then"), but it would be good to investigate and see if we figure out what's going on at some point. If there's some way to have the test suite error instead of hang that would be good, too.

It would also be good to check that the problem is only with the test suite, not with the business logic.