This supports evaluating the script and replying with the result. We
currently serialize JS objects to a string, but we will need to support
dynamic interaction with the objects over IPC. This does not yet support
sending console messages to DevTools.
Some tests take longer than others, and so may want to set a custom
timeout so that they pass, without increasing the timeout for all other
tests. For example, this is done in WPT.
Add an `internals.setTestTimeout(milliseconds)` method that overrides
the test runner's default timeout for the currently-run test.
