t-reftable-pq: add tests for merged_iter_pqueue_top()

t-reftable-pq: add tests for merged_iter_pqueue_top()

merged_iter_pqueue_top() as defined by reftable/pq.{c, h} returns
the element at the top of a priority-queue's heap without removing
it. Since there are no tests for this function in the existing
setup, add tests for the same.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>