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

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

reftable/stack_test.c exercises the functions defined in
reftable/stack.{c, h}. Migrate reftable/stack_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 be in-line with unit-tests'
standards.

Since some of the tests use set_test_hash() defined by
reftable/test_framework.{c, h} but these files are not
'#included' in the test file, copy this function in the
ported test file.

With the migration of stack test to the unit-tests framework,
"test-tool reftable" becomes a no-op. Hence, get rid of everything
that uses "test-tool reftable" alongside everything that is used
to implement it.

While at it, alphabetically sort the cmds[] list in
helper/test-tool.c by moving the entry for "dump-reftable".

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>