Add basic test coverage for the widgets and plugins

Didn't manage to get started on any tests, but sorted out the various phpstan/phpcs issues

Not sure how far you all want to take this, but I guess the something is better than nothing is probably a start. Will try to find time to cover:
- The different Formatters (lazy/non-lazy) at least at a basic level
- The default settings at least at a basic level

Side note: My more selfish goal here is to have enough test coverage to get to a point where you might consider to merge 🐛 Views reference is adding many query string variables to the pager URLs RTBC in as my agency deals with think tanks with 10s of thousands to 100s of thousands of content items and we use this module quite a bit as a foundation for a minimal control of lots of exposed filtering options without needing to know and build VIews (so thank you for the great foundation, just trying to give back a bit where I can).

Note that this contains the code from #3317635: Coding standards → in order for tests to actually pass.

Follow-ups created:

1. 🐛 Add functional coverage (without needing JS), support using the module without JS Needs review
2. 📌 Add test coverage for the views settings plugins Needs work

Merging from upstream after merge of phpcs #3317635: Coding standards → , thanks!

This only adds test coverage which looks pretty good for a first setup. We can always expand/improve later. Merged for now, thanks!

Automatically closed - issue fixed for 2 weeks with no activity.