t/t7704-repack-cruft.sh: clarify wording in --max-cruft-size tests

Now that a number of new tests have landed in t7704, make sure that they
all make sense and are testing the things they say they are.

Things are mostly OK, but a handful of tests needed tweaks. Those tweaks
are as follows:

- Use the terms "too large" or "too small" in tests that exercise the
'--max-cruft-size' behavior. This has historically been treated as a
threshold beneath which to combine cruft packs, but that will change
in a subsequent commit. Prepare for that by using a more generic
term.

- Remove references to "--max-cruft-size" in the freshening tests.
These tests provide coverage of our ability to record updated mtimes
for objects already in cruft packs whose mtimes are upserted from
various sources (loose objects, finding that object in a new pack,
another cruft pack, etc.).

These have nothing to do with the '--max-cruft-size' feature, and in
fact none of the tests even *use* '--max-cruft-size'. Name them
appropriately to make it clear that these tests exercise freshening
behavior, not '--max-cruft-size' behavior.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>