Attributes: Warn & restore boolean attribute & false setter treatment from 3.x

[mgol]

Restore & warn against:

• boolean attributes set to something different than their lowercase names
• boolean attributes queried when set to something different than their lowercase names
• non-boolean non-ARIA attributes set to false

Fixes gh-504
Ref jquery/jquery#5452
Ref jquery/api.jquery.com#1243

[mgol]

This works in jQuery 3.x because - while boolHooks being a fallback for boolean attributes if attrHooks[ name ] didn't exist would not get picked up here - boolHooks only handle a setter and we're in a getter here. The getter - if the get attr hook is missing or returns null - is handled via a call to jQuery.find.attr( elem, name ) which is Sizzle.attr( elem, name ), that defers to Sizzle.selectors.attrHandle, exposed to jQuery via jQuery.expr.attrHandle and that, in turn, is set for boolean attrs by jQuery.

[mgol]

@timmywil I introduced two changes here in a fixup commit:

1. The extraBoolAttrValues check now lowercases the tested value.
2. Tests for extraBoolAttrValues are added. They are only supported when running with jQuery 4.x, but I skipped the warning when running with 3.x (you can see it in added tests) for two reasons:
   1. I didn't want to warn against setting a valid value for which jQuery 4.0.0 adds support.
   2. If someone is setting the hidden value to "until-found" and that gets normalized to "hidden", that's a bug that an upgrade to jQuery 4.x will likely fix.

Let me know what you think.

[mgol]

Landed on both main & 3.x-stable in 5471109; the main patch is tweaked in #555 and the 3.x-stable one in #554.