Don't require ETS_TOOLKIT in tests

[mdickinson]

[PR includes commits from, and extends, #513]

This PR updates our testing machinery in two ways:

• We no longer rely on ETS_TOOLKIT (or QT_API) being set in order to run tests that require a GUI; we just rely on Pyface's default mechanisms for toolkit detection. (As a consequence, we retain the option to use ETS_TOOLKIT if it's helpful.)
• etstool.py support for backends other than those available through EDM has been removed. Meanwhile, support for pyqt6 has been added. (Tests currently fail with PyQt6, due to a known issue in TraitsUI: https://github.com/enthought/traitsui/issues/1926)

Closes #525 Closes #370

[mdickinson]

Merging this into the #513 branch for simplicity. The two branches are too interconnected to work on independently.