Recent changes made it easier to replace the worker configuration for

the duration of a test, but it wasn't perfect. The new dependency

support mutates the configuration, which can impact other tests.
We're now careful to reset the configuration at the start of each test,

to avoid these sorts of issues. We're also more careful to deep-copy

data into the configuration.
I've also renamed default_config to DEFAULT_CONFIG, to better convey

it's intended as a constant and not something mutable.

Unit tests pass.
This resolved some issues hit with upcoming changes.