Add editor tester

In the background page, there could be a button that starts a session with the editor and tests its capabilities:

1. Send `Hello world. Do you see me? Type yes: `
2. Listen to the events until `yes` is typed
3. Send only `Select the word hello on this line`
4. Listen to the selection
5. Send only `Hold the s key for 3 seconds: `
6. Listen to the frequency of events, ensure the events arrive in order, ensure that the cursor always appears at the end
5. Send only `Close this editor tab`

Etc, it could test any number of [editor capabilities](https://github.com/fregante/GhostText/issues/256), including just asking whether something happened a certain way.

Then it could compile a test result

Related:
- https://github.com/fregante/GhostText/issues/183#issuecomment-784261222