MALLOC_CHECK: enable it, unless disabled explicitly

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>

t/perf/perf-lib.sh

@@ -42,7 +42,7 @@ else
 fi
 
 TEST_NO_CREATE_REPO=t
-TEST_NO_MALLOC_=t
+TEST_NO_MALLOC_CHECK=t
 
 . ../test-lib.sh
 

t/test-lib.sh

@@ -96,7 +96,7 @@ export EDITOR
 # Add libc MALLOC and MALLOC_PERTURB test
 # only if we are not executing the test with valgrind
 if expr " $GIT_TEST_OPTS " : ".* --valgrind " >/dev/null ||
-   test -n "TEST_NO_MALLOC_"
+   test -n "$TEST_NO_MALLOC_CHECK"
 then
 	setup_malloc_check () {
 		: nothing