test-temp-directory should always return the same value

[cgay]

Currently test-temp-directory creates and returns a different directory each time it's called, if not called in the same second. At least within the same test body it should always return the same value.

Also, there really should be only one dated directory per test run and then subdirectories for the tests, to avoid directory proliferation. I think we mostly get away with this now because (a) I'm the only one using this function and (b) most of my test suites have very short run time.

[cgay]

I meant to mention the obvious work-around is let test-dir = test-temp-directory(); so this is low priority.