Add webdriver tests for `field-angle` and `field-slider`

[BenHenning]

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Component

field-angle and field-slider

Problem

As #2521 indicates there is need for webdriver tests for at least field-slider and field-angle.

Request

Introduce webdriver tests for #2521 that would help prevent regressions like this from returning. Specifically:

• Tests for enabling editing by opening the custom field editors via click.
• Tests for verifying that the field can be changed both through the inline field editor and the custom drop-down editor.
• Tests for verifying that both the drop-down editor and inline editor sync their states (per #2520 there's even another part of this that could be tested for specific synchronizing order, as well).

It would be prudent to also verify that at least some of these tests fail without the fix from #2521 enabled to ensure they actually do catch the original regression.

There are likely a bunch of other useful tests that could be added with a fully working DOM environment, but this seems like a solid start.

Alternatives considered

Trying to make the new tests needed work with JSDom in Node.js, but this is especially tricky since much of the DOM flows are faked through JSDom.

Additional context

Possibly related to #966.