bug#12206: test-parse-datetime regression test failure with glibc-2.16.0

[Paul Eggert]

It looks like you're running in a setup where 'date'
observes UTC, as opposed to POSIX where 'date' is
supposed to ignore leap seconds.  This test case
assumes the POSIX behavior.  How can we arrange
for the POSIX behavior for the test?  For example,
if we set TZ='EST5' in the environment, are leap
seconds ignored as POSIX says they should be?  If so,
we can arrange to run the test in that environment.