t: move reftable/merged_test.c to the unit testing framework

t: move reftable/merged_test.c to the unit testing framework

reftable/merged_test.c exercises the functions defined in
reftable/merged.{c, h}. Migrate reftable/merged_test.c to the unit
testing framework. Migration involves refactoring the tests
to use the unit testing framework instead of reftable's test
framework and renaming the tests according to unit-tests' naming
conventions.

Also, move strbuf_add_void() and noop_flush() from
reftable/test_framework.c to the ported test. This is because
both these functions are used in the merged tests and
reftable/test_framework.{c, h} is not #included in the ported test.

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>