tests: add date printing and parsing tests

tests: add date printing and parsing tests

Until now, there was no coverage of relative date printing
or approxidate parsing routines (mainly because we had no
way of faking the "now" time for relative date calculations,
which made consistent testing impossible).

This new script tries to exercise the basic features of
show_date and approxidate. Most of the tests are just "this
obvious thing works" to prevent future regressions, with a
few exceptions:

  - We confirm the fix in 607a9e8 that relative year/month
    dates in the latter half of a year round correctly.

  - We confirm that the improvements in b5373e9 and 1bddb25
    work.

  - A few tests are marked to expect failure, which are
    regressions recently introduced by the two commits
    above.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>