Describe the issue

Currently, we have 0 test coverage for widgets like TextBox.
Other widgets are tested here: https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/tests/test_widgets.py

Summary
Following up from #20367 (comment), there might exist more widgets that aren't tested at all, but TextBox seems likely to be used by a substantial audience, so might be worth the attention.

Proposed fix

Simple tests could involve testing set_val, stop_typing, callbacks like on_text_change, on_submit, etc could be a good starting point.