integration-test: Use mock polkitd

integration-test: Use mock polkitd

Add src/tests/test_polkitd.py which provides a simple mock polkitd on a given
bus, as well as some convenience API. This allows test cases to precisely
control which actions get permitted and forbidden without having to assume
anything about the system polkit configuration, users, active/inactive consoles
etc. This is meant to go into polkit itself at some point.

Simplify integration-test to use test_polkitd and get rid of the uid changing
and calling of udisksctl.