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>

Author: Chandra Pratap

Makefile

@@ -1341,6 +1341,7 @@ UNIT_TEST_PROGRAMS += t-oidtree
 UNIT_TEST_PROGRAMS += t-prio-queue
 UNIT_TEST_PROGRAMS += t-reftable-basics
 UNIT_TEST_PROGRAMS += t-reftable-merged
+UNIT_TEST_PROGRAMS += t-reftable-readwrite
 UNIT_TEST_PROGRAMS += t-reftable-record
 UNIT_TEST_PROGRAMS += t-strbuf
 UNIT_TEST_PROGRAMS += t-strcmp-offset
@@ -2682,7 +2683,6 @@ REFTABLE_OBJS += reftable/writer.o
 REFTABLE_TEST_OBJS += reftable/block_test.o
 REFTABLE_TEST_OBJS += reftable/dump.o
 REFTABLE_TEST_OBJS += reftable/pq_test.o
-REFTABLE_TEST_OBJS += reftable/readwrite_test.o
 REFTABLE_TEST_OBJS += reftable/stack_test.o
 REFTABLE_TEST_OBJS += reftable/test_framework.o
 REFTABLE_TEST_OBJS += reftable/tree_test.o

reftable/reftable-tests.h

@@ -13,7 +13,6 @@ int basics_test_main(int argc, const char **argv);
 int block_test_main(int argc, const char **argv);
 int pq_test_main(int argc, const char **argv);
 int record_test_main(int argc, const char **argv);
-int readwrite_test_main(int argc, const char **argv);
 int stack_test_main(int argc, const char **argv);
 int tree_test_main(int argc, const char **argv);
 int reftable_dump_main(int argc, char *const *argv);

t/helper/test-reftable.c

@@ -8,7 +8,6 @@ int cmd__reftable(int argc, const char **argv)
 	block_test_main(argc, argv);
 	tree_test_main(argc, argv);
 	pq_test_main(argc, argv);
-	readwrite_test_main(argc, argv);
 	stack_test_main(argc, argv);
 	return 0;
 }