Add test coverage for the views settings plugins

This adds test coverage for Pager only so far. Going to park here until the chain of merging is completed. Ie merging:

1. #3317635: Coding standards →
2. #3161731: Add basic test coverage for the widgets and plugins →
3. 🐛 Add functional coverage (without needing JS), support using the module without JS Needs review

scott_euser → changed the visibility of the branch 3453902-settings-test-coverage to hidden.

Okay test coverage for all built in Views Reference Settings now. Requires merging 🐛 Add functional coverage (without needing JS), support using the module without JS Needs review first though as these tests rely on Views Reference being usable without JS.

Test only MR, so merging on my own as discussed. Thanks!

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