Convert kasp default test cases to pytest

Convert kasp default test cases to pytest

This commit deals with converting the test cases related to the default
dnssec-policy.

This requires a new method 'check_update_is_signed'. This method will
be used in future tests as well, and checks if an expected record is
in the zone and is properly signed.

Remove the counterparts for the newly added test from the kasp shell
tests script.