Extend UITester support for DirectoryEditor

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

The DirectoryEditor depends on FileEditor, so the testing support for DirectoryEditor depends on testing support for FileEditor.

Note some features of the custom DirectoryEditor can be tested in a similar way compared to the TreeEditor as well.

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.