tests: notice valgrind error in test_must_fail

We tell valgrind to return 126 if it notices that something is wrong,
but we did not actually handle this in test_must_fail, leading to
false negatives.  Catch and report it.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -536,6 +536,9 @@ test_must_fail () {
 	elif test $exit_code = 127; then
 		echo >&2 "test_must_fail: command not found: $*"
 		return 1
+	elif test $exit_code = 126; then
+		echo >&2 "test_must_fail: valgrind error: $*"
+		return 1
 	fi
 	return 0
 }