reftable/stack: extract function to setup stack with N tables

reftable/stack: extract function to setup stack with N tables

We're about to add two tests, and both of them will want to initialize
the reftable stack with a set of N tables. Introduce a new function that
handles this and refactor existing tests that use such a setup to use
it.

Note that this changes the exact records contained in the preexisting
tests. This is fine though as we only care about the shape of the stack
here, not the shape of each table.

Furthermore, with this change we now start to disable auto compaction
when writing the tables, as otherwise we might not end up with the
expected amount of new tables added. This also slightly changes the
behaviour of these tests, but the properties we care for remain intact.

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