test-lib: have SANITIZE=leak imply TEST_NO_MALLOC_CHECK

test-lib: have SANITIZE=leak imply TEST_NO_MALLOC_CHECK

Since 131b94a10a7 (test-lib.sh: Use GLIBC_TUNABLES instead of
MALLOC_CHECK_ on glibc >= 2.34, 2022-03-04) compiling with
SANITIZE=leak has missed reporting some leaks. The old MALLOC_CHECK
method used before glibc 2.34 seems to have been (mostly?) compatible
with it, but after 131b94a10a7 e.g. running:

	TEST_NO_MALLOC_CHECK=1 make SANITIZE=leak test T=t6437-submodule-merge.sh

Would report a leak in builtin/commit.c, but this would not:

	TEST_NO_MALLOC_CHECK= make SANITIZE=leak test T=t6437-submodule-merge.sh

Since the interaction is clearly breaking the SANITIZE=leak mode,
let's mark them as explicitly incompatible.

A related regression for SANITIZE=address was fixed in
067109a5e7d (tests: make SANITIZE=address imply TEST_NO_MALLOC_CHECK,
2022-04-09).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>