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

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

reftable/readwrite_test.c exercises the functions defined in
reftable/reader.{c,h} and reftable/writer.{c,h}. Migrate
reftable/readwrite_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 to
align with unit-tests' naming conventions.

Since some tests in reftable/readwrite_test.c use the functions
set_test_hash(), noop_flush() and strbuf_add_void() defined in
reftable/test_framework.{c,h} but these files are not #included
in the ported unit test, copy these functions in the new test file.

While at it, ensure structs are 0-initialized with '= { 0 }'
instead of '= { NULL }'.

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>