Add E2E Tests

[leonvogt]

It would be awesome to have some End-to-End tests.
Strangely, this isn't as easy as one might think, at least not with the current setup of this extension, which uses a popup to configure the active features.
I tried a couple of times in the past and always failed at some point.

A simple test could look like:

1. Open a headless browser that has this extension installed
2. Navigate to a local HTML file, which has a Turbo Frame inside it
3. Open the popup and active the "Highlight Frames" feature
4. Check if the Turbo Frame has an outline

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The main difficulty is point three, opening the popup in the right context.
An alternative approach could be to not open the popup at all, and instead configure the activated features by mocking the Devtool.getOptions call.