timer: Introduce osmo_clock_gettime to override clock_gettime

Change-Id: I5bebc6e01fc9d238065bc2517058f0ba85620349
diff --git a/tests/timer/clk_override_test.ok b/tests/timer/clk_override_test.ok
new file mode 100644
index 0000000..1fb519c
--- /dev/null
+++ b/tests/timer/clk_override_test.ok
@@ -0,0 +1,8 @@
+Non implemented clocks work fine
+Monotonic clock is working fine by default
+Monotonic clock override enabled
+Monotonic override is cleared by default
+Monotonic clock can be overriden
+osmo_clock_override_add works fine.
+Monotonic clock override disabled
+Monotonic clock is working fine after enable+disable.