tests/extmod: Add tests for sleep_ms/us(), ticks_ms/us/diff().

Simple smoke tests, mostly for coverage. Added to extmod based on the fact
that they're extensions to standard modules.

tests/extmod/time_ms_us.py

+import sys
+import utime
+try:
+    utime.sleep_ms
+except AttributeError:
+    print("SKIP")
+    sys.exit()
+
+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)

tests/extmod/time_ms_us.py.exp

+True
+True