MALLOC_CHECK: Allow checking to be disabled from config.mak

MALLOC_CHECK: Allow checking to be disabled from config.mak

The malloc checks can be disabled using the TEST_NO_MALLOC_CHECK
variable, either from the environment or command line of an
'make test' invocation. In order to allow the malloc checks to be
disabled from the 'config.mak' file, we add TEST_NO_MALLOC_CHECK
to the environment using an export directive.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>