moss/boulder: Add tests and fixtures

[ermo]

Ikey Doherty
Ok lads I've been mulling this a couple days as I knew it couldn't be a one off
Tldr we need to now set a minimum code coverage target and enable coverage reporting/ testing
We also need behaviour testing
So my plan effectively is to bolt a test suite into the tree to drive the built cli ...
We can abuse a few tricks to generate fake .so files and fake .stone's and ship them in the moss git
Ie tiny files but power the index and dep system properly rather than emulated
This way we can test changing of providers, sync, transient deps, and the soname-after-revert issue
The tests will be all offline tests atm
But we will also need to add tests around http, ssl cert validation etc in time
And be able to actively prove robustness and PR correctness