Add more unit tests

[donyunardi]

Feature description

Ran a AV dry-run for teal.widgets:

Consistency checks failed:
 - requirement #0.06 'Nested Closeable Modal Popup' is not covered by any
   test
 - requirement #0.10 'Panel group widget' is not covered by any test
 - requirement #0.11 'Panel item widget' is not covered by any test

Please add unit tests for these items

Code of Conduct

• I agree to follow this project's Code of Conduct.

Contribution Guidelines

• I agree to follow this project's Contribution Guidelines.

Security Policy

• I agree to follow this project's Security Policy.

[donyunardi]

These functions are deprecated. Need to confirm that it is okay to move forward to AV.