t4200: parameterize "rerere gc" custom expiry test

t4200: parameterize "rerere gc" custom expiry test

The test creates a rerere database entry that is two days old, and
tries to expire with three different custom expiry configuration
(keep ones less than 5 days old, keep ones used less than 5 days
ago, and expire everything right now).

We'll be introducing a different way to spell the same "5 days" and
"right now" parameter in a later step; parameterize the test to make
it easier to test the new spelling when it happens.

Signed-off-by: Junio C Hamano <gitster@pobox.com>