test_formatdate(), test_formatdate_zoneoffsets(): Two changes. First,

test_formatdate(), test_formatdate_zoneoffsets(): Two changes.  First,
use the correct way to test for epoch, by looking at the year
component of gmtime(0).  Add clause for Unix epoch and Mac epoch (Tim,
what is Windows epoch?).

Also, get rid of the strptime() test, it was way too problematic given
that strptime() is missing on many platforms and issues with locales.
Instead, simply test that formatdate() gets the numeric timezone
calculation correct for the altzone and timezone.