tests/extmod/time_ms_us: Add test for calling ticks_cpu().

This is just to test that the function exists and returns some kind of
valid value.  Although this file is for testing ms/us functions, put the
ticks_cpu() test here so not to add a new test file.

tests/extmod/time_ms_us.py

@@ -9,3 +9,6 @@ utime.sleep_ms(1)
 utime.sleep_us(1)
 print(utime.ticks_diff(utime.ticks_ms(), utime.ticks_ms()) <= 1)
 print(utime.ticks_diff(utime.ticks_us(), utime.ticks_us()) <= 500)
+
+# ticks_cpu may not be implemented, at least make sure it doesn't decrease
+print(utime.ticks_diff(utime.ticks_cpu(), utime.ticks_cpu()) >= 0)

tests/extmod/time_ms_us.py.exp

@@ -1,2 +1,3 @@
 True
 True
+True