test-lib: declare local variables as local

131b94a10a ("test-lib.sh: Use GLIBC_TUNABLES instead of MALLOC_CHECK_ on
glibc >= 2.34", 2022-03-04) introduced "local" variables without
declaring them as such. This conflicts with their use in some tests (at
least when running them with dash), leading to test failures in:

t0006-date.sh
t2002-checkout-cache-u.sh
t3430-rebase-merges.sh
t4138-apply-ws-expansion.sh
t4124-apply-ws-rule.sh

Declare those variables as local to let the tests pass again.

Signed-off-by: Michael J Gruber <git@grubix.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 8e59c58..cf3fa45 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -516,6 +516,8 @@
 	}
 else
 	setup_malloc_check () {
+		local g
+		local t
 		MALLOC_CHECK_=3	MALLOC_PERTURB_=165
 		export MALLOC_CHECK_ MALLOC_PERTURB_
 		if _GLIBC_VERSION=$(getconf GNU_LIBC_VERSION 2>/dev/null) &&