rtc: rtctest: Improve support detection

The rtc-generic and opal-rtc are failing to run this test as they do not
support all the features. Let's treat the error returns and skip to the
following test.

Theoretically the test_DATE should be also adjusted, but as it's enabled
on demand I think it makes sense to fail in such case.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>