Time tests fail sporadically on Mac

[catamorphism]

When running make check on my Mac, three tests in time::tests (test_at, test_ctime, and test_strftime) sometimes fail. Usually if I just run make check again, the tests pass.

[brson]

my suspicion is that the calls to the C tzset function access the environment, which is especially racy on os x, causing bogosity.

[catamorphism]

test_strptime is also failing now, possibly for the same reason (it also calls tzset).

[catamorphism]

Fixed by @erickt in 9898485