Extend UITester support for HistoryEditor

The goal would be to extend support for downstream projects to write tests that require interacting with the GUI objects created by HistoryEditor.

Actions:
(1) Add (toolkit agnostic) tests for the editor in TraitsUI, focus on interactions that are going to be useful downstream as well
(2) Expand the testing support via the default target registry used by UITester

Optional and opportunistic: See if a test can be added for the demo using this editor (if there is one), related to #1257. It might also provide motivation as to what features are likely useful downstream.

Start by examining HistoryEditor interactions in TraitsUI and the default target registry used by UITester. Add toolkit-agnostic editor tests and extend registry support for downstream GUI tests; optionally inspect the demo and related issue #1257 for an additional test opportunity.

Written by the indexing model from the issue text.