The malloc checks in tests are currently disabled.  Actually evaluate
the variable for turning them off and enable them if it's unset.

Also use this opportunity to give it the more descriptive and
consistent name TEST_NO_MALLOC_CHECK.
Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>