leak tests: mark "sort" test as passing SANITIZE=leak

Mark a test that was recently added in e031e9719d (test-mergesort:
add test subcommand, 2021-10-01) as passing with SANITIZE=leak. It
will now be listed as running under the
"GIT_TEST_PASSING_SANITIZE_LEAK=true" test mode (the "linux-leaks" CI
target).

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

t/t0071-sort.sh

@@ -2,6 +2,7 @@
 
 test_description='verify sort functions'
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 test_expect_success 'llist_mergesort()' '