Add testing helpers

[Julian]

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 (regret.testing should provide a testing helper for ensuring unexpected deprecations fail tests #28)
• Turn all deprecations into exceptions

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