osdep/timer: update documentation and test for mp_time_ns

No longer true after 8bbcc87.

osdep/timer.h

@@ -23,7 +23,7 @@
 // Initialize timer, must be called at least once at start.
 void mp_time_init(void);
 
-// Return time in nanoseconds. Never wraps. Never returns 0 or negative values.
+// Return time in nanoseconds. Never wraps. Never returns negative values.
 int64_t mp_time_ns(void);
 
 // Return time in seconds. Can have down to 1 nanosecond resolution, but will

test/timer.c

@@ -13,7 +13,7 @@ int main(void)
     /* timekeeping */
     {
         int64_t now = mp_time_ns();
-        assert_true(now > 0);
+        assert_true(now >= 0);
 
         mp_sleep_ns(MP_TIME_MS_TO_NS(10));