t-reftable-stack: add test for stack iterators

t-reftable-stack: add test for stack iterators

reftable_stack_init_ref_iterator and reftable_stack_init_log_iterator
as defined by reftable/stack.{c,h} initialize a stack iterator to
iterate over the ref and log records in a reftable stack respectively.
Since these functions are not exercised by any of the existing tests,
add a test for them.

Mentored-by: Patrick Steinhardt <redacted>
Mentored-by: Christian Couder <redacted>
Signed-off-by: Chandra Pratap <redacted>
Signed-off-by: Junio C Hamano <redacted>