Extend UITester support for CSVEditor

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

At a glance, we should be able to simply reuse the testing support for TextEditor.

Actions:
(1) Expand the testing support via the default target registry used by UITester, maybe by reusing logic implemented for testing TextEditor
(2) Rewrite the existing tests for CSVEditor to use UITester (to test the API).
(3) Add more tests to exercise any new testing support in TraitsUI's own test suite for CSVEditor

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.