t/test-lib.sh: fix running tests with --valgrind

We forgot to adjust this code path after moving the test helpers to
t/helper/.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: dscho

t/test-lib.sh

@@ -785,7 +785,7 @@ then
 	# override all git executables in TEST_DIRECTORY/..
 	GIT_VALGRIND=$TEST_DIRECTORY/valgrind
 	mkdir -p "$GIT_VALGRIND"/bin
-	for file in $GIT_BUILD_DIR/git* $GIT_BUILD_DIR/test-*
+	for file in $GIT_BUILD_DIR/git* $GIT_BUILD_DIR/t/helper/test-*
 	do
 		make_valgrind_symlink $file
 	done