pytests: add basic test for the m41t80 rtc

as we test for it being rtc0 this basically assumes that it's the only rtc
too.

pytests/test_rtc.py

+def test_rtc0_exists():
+    with open('/sys/class/rtc/rtc0/name') as f:
+        assert('rtc-m41t80' in f.readline().strip())