regret.testing should provide a testing helper for ensuring unexpected deprecations fail tests

Right now `Recorder` doesn't necessarily have a way to ensure that deprecations are explicitly "expected", so a `Deprecator` can call `Recorder.emit` and if the test case does not check it, nothing will complain (c.f. how `trial.unittest.TestCase` will fail tests that do not flush warnings).

There should be a testing helper provided which can be used to ensure that any deprecation must have `expect` called for it, otherwise a test is failed.

(Perhaps it should then be impossible to do anything *but* expect deprecations in this way -- i.e. impossible to use `Recorder` without accounting for all deprecations).