tests: add some basic tests of phabricator interactions
This uses the vcr library to avoid hitting phabricator on every test
execution. In order to generate new recordings (vcr calls them
cassettes) just remove the appropriate json file, and the test will
regenerate it. It's not my favorite way to test things, but it'll let
us have test coverage on the phabricator extension that'll make it
resilient to refactors in core and let us move it to hgext.
In the future, it'd probably be better to have a docker container we
can spin up for creating the vcr recordings, but for now this is
enough better than nothing I'm going to declare victory.
Coverage reports about 73% of the extension is now covered.
Differential Revision: https://phab.mercurial-scm.org/D4601