Add more unit tests for page components/controls

[huong-li-nguyen]

Description

• Update docs to show how to use Button as a ControlType via custom component approach
• Add unit tests for valid components/controls

Follow-up PR to this: #88

Screenshot

Checklist

• I have not referenced individuals, products or companies in any commits, directly or indirectly
• I have not added data or restricted code in any commits, directly or indirectly
• I have updated the docstring of any public function/class/model changed
• I have added the PR number to the change description in the changelog fragment, e.g. Enable feature XXX ([#1](https://github.com/mckinsey/vizro/pull/1)) (if applicable)
• I have added tests to cover my changes (if applicable)

Types of changes

• Docs/refactoring (non-breaking change which improves codebase)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Notice

• I acknowledge and agree that, by checking this box and clicking "Submit Pull Request":

  • I submit this contribution under the Apache 2.0 license and represent that I am entitled to do so on behalf of myself, my employer, or relevant third parties, as applicable.
  • I certify that (a) this contribution is my original creation and / or (b) to the extent it is not my original creation, I am authorised to submit this contribution on behalf of the original creator(s) or their licensees.
  • I certify that the use of this contribution as authorised by the Apache 2.0 license does not violate the intellectual property rights of anyone else.

[maxschulz-COL]

Lgtm, just 2 suggestions to make the text flow a little more

[huong-li-nguyen]

Lgtm, just 2 suggestions to make the text flow a little more

I like it - thanks! 📝