Convert model2.multisigner test to pytest

Convert model2.multisigner test to pytest

This converts the model2.multisigner tests from the multisigner system
test to pytest based code. Crappy shell test functions such as
'zsks_are_published', 'records_published' and others are replaced with
the standard test code from isctest.kasp and by setting 'private=False'
and 'legacy=True' on the keys from the other providers so we don't do
any key file testing.

(cherry picked from commit 773ce8d99b4c5074e0368e0c1ef2af250b043255)