Support for testing additional accessibility properties beyond name and role.

[jcsteh]

Closes https://github.com/web-platform-tests/interop-accessibility/issues/211

1. Implement get_element_accessible_properties and get_accessible_properties in wptrunner and TestDriver. For wptrunner, this is currently only supported in executormarionette (Gecko), as there is no implementation yet in any other browser to support.
2. Add AriaUtils.verifyAccessibilitySubtree for simpler tree verification using a simple JS object.
3. Add simple WPT tests exercising and demonstrating these new methods.

[jcsteh]

There are a number of blocking details to sort out here, but I figured it was worth posting this to get things moving and get some initial feedback.

1. There is ongoing, unresolved discussion in https://github.com/web-platform-tests/rfcs/pull/226 as to whether these TestDriver methods need to be prefixed with tentative_. In short, RFC 127 just says "an existing WebDriver endpoint", which isn't explicit about a formal spec or where it needs to be specified. If an explainer is sufficient, we have https://github.com/WICG/aom/issues/203.
2. In https://github.com/web-platform-tests/rfcs/pull/226/files#r2411669032, @cookiecrook questioned whether we should be removing accessible children/parent ids from this first pass in order to unblock it sooner. I feel this is a fundamental detail we need to sort out because it is a primary goal of the work here. However, pragmatism is important, so I'm happy to go with the consensus here. I guess that would also involve removing verifyAccessibilityTree from this first pass.
3. executormarionette being the only implementation in this PR is a bit odd. On the other hand, it's difficult to implement (and verify the implementation of) this in executorwebdriver because we don't have a browser implementation yet to use.

[zcorpan]

From our last meeting I think we can omit "tentative" prefixes in the API names and instead assert that the methods are only used in tentative tests. I've implemented that in this PR.

[alice]

(All my comments should be considered optional suggestions! I think this is fine already.)