Add forked field to GitHub repo fixtures
GitHub's API returns a "fork": True/False
field, let's give ourselves a chance to exploit it.
In the meantime, fix a newly uncovered bug
RPC client/server tests: use proper pattern for call assertions
.called_once_with
doesn't exist, so it was just returning a mock
object which was boolean True; however pytest-mock is now catching
this (correctly) as a programming error, so handle the call args of the
exception capture properly.