tests: move t0009-prio-queue.sh to the new unit testing framework

tests: move t0009-prio-queue.sh to the new unit testing framework

t/t0009-prio-queue.sh along with t/helper/test-prio-queue.c unit
tests Git's implementation of a priority queue. Migrate the
test over to the new unit testing framework to simplify debugging
and reduce test run-time. Refactor the required logic and add
a new test case in addition to porting over the original ones in
shell.

Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>