Adding test to timed text editor

[emettely]

Is your Pull Request request related to another issue in this repository ?

Not from this issue, but DPE has a dependency on react-transcript-editor and we want to start adding tests to it.
bbc/digital-paper-edit-electron#4

Describe what the PR does
Adding the most basic test to timed-text-editor and also removing CustomEditor because it's a wrapper component for one single function - we don't really need it.

State whether the PR is ready for review or whether it needs extra work
Ready

Additional context

[pietrop]

removing CustomEditor

You might want to check this change against this issue #159 (which is the reason why it was initially separated into its own component)

see #160 for more details.

TL;DR: using react inspector tool highlight updates you can see if removing CustomEditor as separate component increases the number of unnecessary re-renders. You'll have to judge it based on colour.

[emettely]

A very good catch, the re-renders definitely don't happen for the original master.

vs this branch:

onChange and handleKeyCommand makes the Editor re-render.

[emettely]

facebook/react#16437 (comment) The original highlighting function has disappeared but instead you can use the profiler to record the rendering.

[emettely]

Added back the Editor as a memoized react component, and now the re-renders are gone.