test-temp-directory isn't unique enough

[cgay]

test-temp-directory returns the same directory if called within the same second. It should be unique for each test run. Adding milliseconds could be good enough, but kind of kicks the can down the road. Maybe use a hash instead. It's easy enough to sort by time when looking for the test outputs. Or add -1, -2, etc at the end.

Not generally a problem for users; I noticed it due to a bug in which deft test ran the tests twice in a row.