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

reftable/tree_test.c exercises the functions defined in
reftable/tree.{c, h}. Migrate reftable/tree_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' standards.

Also add a comment to help understand the test routine.

Note that this commit mostly moves the test from reftable/ to
t/unit-tests/ and most of the refactoring is performed by the
trailing commits.

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>

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