core_form: improve support for client-side validation of group rules

As part of the library upgrade in MDL-63070, I've been looking at the vanilla quickforms rule support (yes, I know this is very dated). On comparing this to the support which our MoodleQuickForm wrapper provides, it's clear we've either lost, or were always missing the client-side validation for group rules. In fact, there are some rules supported by the vanilla library which I don't think we ever supported properly, despite advocating the use of the rule APIs in our docs. Additionally, the testing of various form configurations is lacking in Moodle, meaning that other bugs (like MDL-57684) have been introduced over the years, further reducing our level of support.

This issue will aim to address these shortcomings in validation, and should also try to provide a generic forms behat test, validating all configurations of single and group client-side rules that Moodle needs to support.