functional_test.go was also split into one file per resource, to match
what we're doing for the RPC handlers too. See #891
I also needed to make some changes to move the tests to a separate
package:
* `NewAteletDialer` now takes `options`, and `WithDialCredentials` lets
a test build its own transport credentials. The fake atelet is reached
over insecure transport. Didn't change existing callers. This is the
only non-test change.
* Moved a few unit tests that are actually functional tests to the
respective file under functionaltest/
Fixes#891
- [x] Tests pass
- [x] Appropriate changes to documentation are included in the PR
