Add automated testing

[fregante]

Context

The tests run on a few pages and they output their results in the console, which the tester has to manually open to verify:

• background page (to test the CS api)
• a website (to test the background API)
• a website (to test the CS api via the background page) in Implement "to content script" via background page #22
• options page
• … more in the future

Ideally

1. Browser launches via CLI
2. Retrieves the log from each context in series (or separate runs)
3. Outputs it to the CLI
4. Exits with 0 if the tests fail

Prior art

https://github.com/juliangruber/tape-run

I used this before and it's fantastic, but it runs the code autonomously, as a regular page script (we'd need the code to run in extension contexts)

Alternatives

Minimal, complementary, Node-only testing with mocks could be written, but it would not replace the existing tests. This would verify the internals (logging, retries, exact messages)