tests: Add lib/timestamp-test test case

Signed-off-by: Jakub Czapiga <jacz@semihalf.com>
Change-Id: I39abfc644fef085cef2175086a0e45a040b244de
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46968
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>

tests/include/stubs/timestamp.h

@@ -0,0 +1,6 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#include <types.h>
+
+void dummy_timestamp_set(uint64_t v);
+
+void dummy_timestamp_tick_freq_mhz_set(int v);