Re-enable and fix-up test_radio.py

We can re-enable `test_radio.py` as its associated B2G blocker is now fixed with a kernel update for Unagi devices. This pull also adds a power check and tear-down with which we need to turn off the radio on test-end.

Also, see https://github.com/mozilla/gaia-ui-tests/pull/225/ — I'm not sure what we want to do with this test-file as there would be duplicate testing.