test_plugable: update the rest of test_init
In commit ed4c2d9252a995d01dc098e5b761ded8cd9373d8, changes to the Plugin
class were made, but the test was updated only partially.
Update the rest to fix the failing test.
https://fedorahosted.org/freeipa/ticket/6313
Reviewed-By: Martin Basti <mbasti@redhat.com>