Bug 1814506 - Add unit tests to cover multi selection of tabs

[DreVla]

Three new unit tests have been added to cover cases when multiple tabs are selected or unselected.

Pull Request checklist

• [X] Tests: This PR includes thorough tests or an explanation of why it does not
• [ ] Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• [X] Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

QA

• [ ] QA Needed

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[DreVla]

Hey @MozillaNoah I know you are quite busy with tabs tray refactoring and considering that there the code freeze is soon and this is just a simple addition of unit tests, if you have some time, could you please take a quick look so the PR can be merged? Thank you!

[DreVla]

Even though this is approved, since it is code freeze day, we will not land it until after the Fenix migration.

[gabrielluong]

The monorepo team is working to close out opened PRs before the migration and archiving this repo. Since this is just a test change and to help avoid re-applying the changes on the monorepo, I am gonna push this to landing today.