Add start of GUI tests for clippy lints page

[GuillaumeGomez]

This is the beginning of adding GUI tests for the clippy lints page which had a number of regressions recently. I'll add (way) more if this is something the clippy team is interested into.

Now let me explain a bit why I picked this framework: browser-ui-test is meant to make it easy to write and read web GUI tests for people who are not familiar with web development (which is the case for most clippy contributors I suppose). It works by checking states of the various elements in the HTML page.

It also supports screenshots, however this feature is disabled by default because fonts have very small differences in differences in rendering, making screenshot comparisons mostly pointless unless they are all performed on only one computer, which in turn would limit contributors to add or update GUI tests. Even when not rendering fonts, the layout had some small subtle differences which again prevented the screenshots comparison to work.

This is the approach we decided to take for rustdoc and docs.rs and for now it seems to work nicely for both projects.

Don't hesitate if you have more questions.

changelog: Add GUI tests for clippy lints page

r? @flip1995

[GuillaumeGomez]

@flip1995 and I talked about who would be best to review it and it seems only @Alexendoo have front-end development knowledge so assigning them as reviewer to this PR.

r? @Alexendoo

[bors]

☔ The latest upstream changes (presumably #13587) made this pull request unmergeable. Please resolve the merge conflicts.

[GuillaumeGomez]

Fixed merge conflicts.

@Alexendoo: Don't hesitate if you need help for reviewing this PR.

[Alexendoo]

I'm concerned it would be too much of a roadblock for contributors to the site. The tests rely on the page implementation details directly and updating these tests requires installing some pretty heavy dependencies (node/npm/puppeteer), plus learning a new DSL

I would be more comfortable with something like playwright tests since JS is at least familiar and there's auto complete to help out, but I still wouldn't be sure

[GuillaumeGomez]

Well, testing websites will require something like puppeteer in any case. For the rest, seems like pretty good concern. I sadly don't have a better suggestion though as they all come with great cost.

I would have loved to have "something" when we were migrating the clippy lints page though.