Move Tests to Specta/Expecta

[dannygreg]

It's nicer in basically every way.

We should also use the opportunity to consolidate our fixture setup and teardown. My current Diff branch suffers from a bit of schizophrenia as we need to make sure the fixtures are available for our existing (non specta) tests.

[mdiep]

Objective-Git uses Specta/Expecta now.