[Reader IA] Add more tests to Reader Navigation and Filtering Improvements

[thomashorta]

We made several changes when working on the Reader Navigation and Filtering Improvements project and were not able to cover all newly added code with Unit/UI tests.

This GH issue was created to track the task of coming back to it and adding those tests in the future since we were not able to do so at this time due to time constraints.

Feature PR: #19978
Internal Ref: pcdRpT-506-p2

[peril-wordpress-mobile]

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[dangermattic]

Thanks for reporting! 👍