Refactor FXIOS-12135 [Tab tray UI experiment] Enable tab tray experiment in dev & beta builds

[lmarceau]

:scroll: Tickets

Jira ticket Github issue

:bulb: Description

This is the follow up PR on https://github.com/mozilla-mobile/firefox-ios/pull/26878. We are adjusting the final UI tests with the tab tray experiment changes.

:pencil: Checklist

• [X] I filled in the ticket numbers and a description of my work
• [X] I updated the PR name to follow our PR naming guidelines
• [ ] I ensured unit tests pass and wrote tests for new code
• [ ] If working on UI, I checked and implemented accessibility (Dynamic Text and VoiceOver)
• [ ] If adding telemetry, I read the data stewardship requirements and will request a data review
• [ ] If needed, I updated documentation and added comments to complex code
• [ ] If needed, I added a backport comment (example @Mergifyio backport release/v120)

[mobiletest-ci-bot]

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[mergify[bot]]

This pull request has conflicts when rebasing. Could you fix it @lmarceau? 🙏