Add editor tester

[fregante]

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
7. Send only Close this editor tab

Etc, it could test any number of editor capabilities, including just asking whether something happened a certain way.

Then it could compile a test result

Related:

• Never update the local text while the user is typing #183 (comment)