tests: Add proper license headers

Signed-off-by: Jan Dabros <jsd@semihalf.com>
Change-Id: Id8ca7c53122632c674e6bf952046ea22c0408e55
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41088
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

tests/lib/string-test.c

@@ -1,3 +1,6 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/* This file is part of the coreboot project. */
+
 #include <stdarg.h>
 #include <stddef.h>
 #include <setjmp.h>