DiffMerge gets killed on test exit

[martinsson]

DiffMerge was getting killed on my mac. It turns out a hacky fix (https://github.com/martinsson/ApprovalTests.Java/commit/01dc6eaa33904ed0cf6c895aa0810c29791892ab) will to the job.

I'd be happy if someone had a better grasp of why this is so. But if we don't understand perhaps this is a good enough solution?

[nikolavp]

Can you verify if diffmerge has the same problem in my more modern approval java port at (https://github.com/nikolavp/approval)