Pre-create outut folder for Lighthouse reports

[bollwyvl]

continues towards #206.

Sorry to let this slide. Turns out I missed a few steps in the copy-and-paste of the templates, and the outputFolder needs to already exist. This would apparently fix, for example, this run:
https://github.com/pandas-dev/pydata-sphinx-theme/runs/805676569?check_suite_focus=true

However, the timeout error still eludes me... but maybe we can start seeing some reports, some of the time!

[jorisvandenbossche]

So it seems there now is a report (sometimes it times out, but often it succeeds): https://github.com/pandas-dev/pydata-sphinx-theme/actions/runs/147113570

But when checking the report, it seems it got an error loading the page ..

Lighthouse was unable to reliably load the page you requested. Make sure you are testing the correct URL and that the server is properly responding to all requests. (Status code: 404)

[bollwyvl]

So maybe instead of curl, I should try wget --mirror --retry-on-http-error=404 which will crawl the whole page, and presumably fail if any of the assets aren't available.

Or, and this would likely put it back in a testable-per-PR state, perhaps testing it against a locally running server would be sufficient... there would be a number of false alarms (e.g. no https, http/2, and probably some cross-domain gotchas for localhost) but for the accessibility stuff, it might be completely sufficient... i'll investigate that path...