Create a `testing` submodule

[rwb27]

We now have a few bits of code scattered through the package for testing, like create_thing_without_server and friends. It would be nice to consolidate these in a module that makes it obvious they're there for testing purposes. I propose we create a new submodule called testing for these symbols.

The testing submodule could also expose a few example Things that are useful for test purposes. Some of the example Things in the package should probably move into the tests folder instead, though.

[rwb27]

@julianstirling did this in #199