force mocha tests to exit to avoid hangs

[campersau]

There are some CI cancellations now because of tests hangs:

• https://github.com/FredrikNoren/ungit/runs/685921173?check_suite_focus=true
• https://ci.appveyor.com/project/FredrikNoren/ungit/builds/32943552/job/m2anlaiuae4owaw8

So we might need to add --exit for now because I haven't reproduced these hangs yet.

https://github.com/FredrikNoren/ungit/pull/1365#issuecomment-630683583