Add testing helpers

This would / should be used by downstream packages who:

- [ ] Are doing deprecations themselves and want to test code that should be deprecated
- [ ] Potentially fail for deprecated code that should have been removed (e.g. where `version` is later than the detected current version)
- [ ] Are *not* doing deprecations themselves and want to ensure other code they run does not raise deprecations (#28)
- [ ] Turn all deprecations into exceptions

(Possibly those are different and should split into separate tickets).