The Firefox View tests, i.e. `browser/components/firefoxview/tests/browser/browser_opentabs_pinned_tabs.js:test_mute_unmute_pinned_tab` sends [a click on a pinned mute/unmute control](https://searchfox.org/mozilla-central/rev/b55a24116575b1b6c17f1aa56a0b142d86dfa41c/browser/components/firefoxview/tests/browser/browser_opentabs_pinned_tabs.js#324-325,341-342) that is intentionally made not focusable to follow the existent tab panel behavior. A keyboard-only user could mute/unmute this pinned tab via the context menu, while we do not want to create an additional, unnecessary tabstop for this control as well. Thus we add an exception from the a11y_checks for this tests via `setEnv` and remove the `fail-if` notation from its test manifest. Depends on D203987 Differential Revision: https://phabricator.services.mozilla.com/D203988