Fix unit test interdependence

[huong-li-nguyen]

Description

• Fix unit test interdependence caused by dash.page_registry not being cleaned up after dashboard.build
  • Replace app.build with dashboard.build in unit tests for Accordion / Navigation /Dashboard
  • Add unit test test_page_build_left_side_removed (could previously not be added due to interdependence)
• Refactor Accordion model (as part of an investigation on whether we can get rid of the dashboard.build dependence)

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

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[maxschulz-COL]

Lgtm! Thanks for tackling this

[petar-qb]

I really like the refactored code. It's so easy to read and understand now.

[maxschulz-COL]

I like the new idea of moving it to the validator, this improved the code readibility a lot! And we can remove the registry access in one place only if we ever want to

[petar-qb]

I shared a little note here, but feel free to overlook it and go ahead with the merge if it doesn't quite resonate with you 😄.