t/helper: refactor to not use `struct reftable_table`

t/helper: refactor to not use `struct reftable_table`

The `struct reftable_table` interface in our "reftable" test helper gets
used such that we can easily print either a single table, or a merged
stack. This generic interface is about to go away.

Prepare the code for this change by using merged tables instead. When
printing the stack we've already got one. When using a single table, we
can create a merged table from it to adapt.

This removes the last user of the generic interface.

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