tests: fix a memory leak in test-oidtree.c

tests: fix a memory leak in test-oidtree.c

Fix a memory leak in t/helper/test-oidtree.c, we were not freeing the
"struct strbuf" we used for the stdin input we parsed. This leak has
been here ever since 92d8ed8ac10 (oidtree: a crit-bit tree for
odb_loose_cache, 2021-07-07).

Now that it's fixed we can declare that t0069-oidtree.sh will pass
under GIT_TEST_PASSING_SANITIZE_LEAK=true.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>