pretty tests: give --date/format tests a better description

pretty tests: give --date/format tests a better description

Change the description for the --date/format equivalency tests added
in 466fb6742d7 (pretty: provide a strict ISO 8601 date format,
2014-08-29) and 0df621172d8 (pretty: provide short date format,
2019-11-19) to be more meaningful.

This allows us to reword the comment added in the former commit to
refer to both tests, and any other future test, such as the in-flight
--date=human format being proposed in [1].

1. http://lore.kernel.org/git/pull.939.v2.git.1619275340051.gitgitgadget@gmail.com

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>