Add WPT for `InputEvent.getTargetRanges()

There are no automated tests for `InputEvent.getTargetRanges()` because it
is set only when `beforeinput` event, but it's defined as not dispatched by
`document.execCommand`.

However, we can synthesize `beforeinput` event with test driver.

On the other hand, the definition in Input Events spec is not clear.
Therefore, most of the tests won't be passed on any browsers for now.
There are some spec issues which I filed:
* w3c/input-events#112
* w3c/input-events#113
* w3c/input-events#114

These new test must be useful when browser vendors discuss the issues.

Differential Revision: https://phabricator.services.mozilla.com/D85527

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1655393
gecko-commit: 12f21ad909371384939bb38d65cf1dbc797b5bf9
gecko-integration-branch: autoland
gecko-reviewers: smaug