environs/testing: move FakeHome etc from testing

environs/testing: move FakeHome etc from testing

All the fake home stuff is about setting up environments.
Given that environments/testing exists, it makes sense
for it to live there.

There's one unfortunate wrinkle, which is that one
state internal test suite now depends on environs/testing,
which depends on environs, which depends on state.
The dependency is resolved with a moderately
sleazy variable initialisation.

I still think the move is worth it, despite the above.

https://code.launchpad.net/~rogpeppe/juju-core/383-move-fakehome-etc/+merge/183243

(do not edit description out of merge proposal)