Tests: make all classes either abstract or final

[jrfnl]

Context

• Code QA/consistency

Summary

This PR can be summarized in the following changelog entry:

• Code QA/consistency

Relevant technical choices:

*

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• N/A No functional changes.

Relevant test scenarios

• [ ] Changes should be tested with the browser console open
• [ ] Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies

[coveralls]

Pull Request Test Coverage Report for Build 7229198258

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 50.203%

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Totals
Change from base Build 7229177020:	0.0%
Covered Lines:	1236
Relevant Lines:	2462

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💛 - Coveralls