[23.0] Tool filtering fix for label handling

[dannon]

Better testing of whether or not a thing is a tool for toolbox normalzation/flattening.

Fixes #16164

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. Add a toplevel label or point your client at usegalaxy.fr; search.
  2. I'll add a test in the morning.

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[mvdbeek]

Thank you!