Tests for Spicedham

Tests for all methods in `spicedham/__init__.py`
@willkg r?
Looks like this gets repeated a lot. I have similarish messy patch code in fjord that I abstracted into a decorator.

https://github.com/mozilla/fjord/blob/master/fjord/translations/tests/test_gengo.py#L44

Those two decorators take arguments, so they're a bit more complicated than what I think you need here.