tests: make SANITIZE=address imply TEST_NO_MALLOC_CHECK

tests: make SANITIZE=address imply TEST_NO_MALLOC_CHECK

As the address sanitizer checks for a superset of the issues detected
by setting MALLOC_CHECK_ (which tries to detect things like double
frees and off-by-one errors) there is no need to set the latter when
compiling with -fsanitize=address.

This fixes a regression introduced by 131b94a10a ("test-lib.sh: Use
GLIBC_TUNABLES instead of MALLOC_CHECK_ on glibc >= 2.34", 2022-03-04)
which causes all the tests to fail with the message

    ASan runtime does not come first in initial library list;
    you should either link runtime to your application or
    manually preload it with LD_PRELOAD.

when git is compiled with SANITIZE=address on systems with glibc >=
2.34. I have tested SANITIZE=leak and SANITIZE=undefined and they do
not suffer from this regression so the fix in this patch should be
sufficient.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>