t/unit-tests: introduce reftable library

t/unit-tests: introduce reftable library

We have recently migrated all of the reftable unit tests that were part
of the reftable library into our own unit testing framework. As part of
that migration we have duplicated some of the functionality that was
part of the reftable test framework into each of the migrated test
suites. This was a sensible decision to not have all of the migrations
dependent on each other, but now that the migration is done it makes
sense to deduplicate the functionality again.

Introduce a new reftable test library that hosts some shared code and
adapt tests to use it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>