Extracted from `CrossFrame` functionality that was related to the
injection of the Hypothesis client into its own class,
`HypothesisInjector`.

In addition, I simplified and modernised the `multi-frame-test.js`.

There are a couple of follow up questions:

- should the utility functions in `frameUtil` be consolidated in the
  `hypothesis-injector.js` file? Most of this functionality is only used
  in the `HypothesisInjector` or `FrameObserver` classes.

- should `HypothesisInjector` be combined with `FrameObserver`?