t/t6500-gc.sh: refactor cruft pack tests

t/t6500-gc.sh: refactor cruft pack tests

In 12253ab6d0 (gc: add tests for --cruft and friends, 2022-10-26), we
added a handful of tests to t6500 to ensure that `git gc` respected the
value of `--cruft` and `gc.cruftPacks`.

Then, in c695592850 (config: let feature.experimental imply
gc.cruftPacks=true, 2022-10-26), another set of similar tests was added
to ensure that `feature.experimental` correctly implied enabling cruft
pack generation (or not).

These tests are similar and could be consolidated. Do so in this patch
to prepare for expanding the set of command-line invocations that enable
or disable writing cruft packs. This makes it possible to easily test
more combinations of arguments without being overly repetitive.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>