tests/extmod: Fix access of RTC class in machine.RTC test.

This previously passed on some targets that automatically import the
`machine` module in `boot.py`.

Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/machine_rtc.py

@@ -6,7 +6,7 @@ except ImportError:
     print("SKIP")
     raise SystemExit
 
-rtc = machine.RTC()
+rtc = RTC()
 
 # Save datetime.
 orig_datetime = rtc.datetime()