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/stubs/timestamp.c

@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include "stubs/timestamp.h"
+
+static uint64_t timestamp_value = 0;
+static int timestamp_tick_freq_mhz_value = 1;
+
+/* Provides way to control timestamp value */
+void dummy_timestamp_set(uint64_t v)
+{
+	timestamp_value = v;
+}
+
+/* Provides way to control timestamp tick frequency MHz value */
+void dummy_timestamp_tick_freq_mhz_set(int v)
+{
+	timestamp_tick_freq_mhz_value = v;
+}
+
+/* Reimplementation of timestamp getter to control behaviour */
+uint64_t timestamp_get(void)
+{
+	return timestamp_value;
+}
+
+int timestamp_tick_freq_mhz(void)
+{
+	return timestamp_tick_freq_mhz_value;
+}