[Infrastructure] - Create a WidgetTester extension to drag scroll bars

[matthew-carroll]

In https://github.com/superlistapp/super_editor/pull/1649 we added a few tests that drag a scroll bar. This requires hovering over the scroll bar, waiting for it to appear, and then dragging and releasing it.

Let's add a tool to flutter_test_robots to coordinate this behavior, then, let's use that tool in super_editor instead of manually implementing that.

PS - Please copy the relevant details from this ticket over to a flutter_test_robots issue.

[angelosilvestre]

I PR is already opened in the test robots repo: https://github.com/Flutter-Bounty-Hunters/flutter_test_robots/pull/23