tests: AIX can't handle negative date in test-dirstate.t
test-dirstate.t fails on AIX in the absurd date test. AIX touch errors on
any date prior to 1970. AIX mktime() gives an error on such dates, so the
problem is deeper than touch and attempts to work around touch in Python
failed.
Give up. Add an AIX test to hghave and skip the absurd date test on AIX.
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