Bug 1854447 - Temporarily fail-if some Tier 2 a11y_checks for Desktop Firefox. r=Gijs

We are working on [enabling Tier 2 a11y-checks to ensure our products have basic accessibility built-in (bug 1692110)](https://bugzilla.mozilla.org/show_bug.cgi?id=1692110) and before that, we need to prepare the existing code base. While we triage and investigate existent tests that would fail once the a11y-checks are enabled in the CI, we are going to fail-if the failing tests altogether and then handle each one of them individually.

This task is to add `fail-if` condition of the failing tests while we continue investigation into the reasons these tests failed and to be able to backtrack these tests once they're resolved.

Related bugs:
- Disabling tests: meta bug 1848402
- Investigation and remediation for components with disabled tests: meta bugs 1848394 and 1854227, component-specific meta bug 1854448
- Explore capturing click events for a11y_checks: bug 1692110

Depends on D188910

Differential Revision: https://phabricator.services.mozilla.com/D188911

toolkit/components/aboutconfig/test/browser/browser.toml

@@ -11,8 +11,10 @@ support-files = ["head.js"]
 ["browser_basic.js"]
 
 ["browser_clipboard.js"]
+fail-if = ["a11y_checks"] # Bug 1854447 th, td.cell-value, #prefs may not be focusable; #show-all may be unlabeled
 
 ["browser_edit.js"]
+fail-if = ["a11y_checks"] # Bug 1854447 span, th, td.cell-value may not be focusable; #show-all, input, button-add/delete/reset ghost-buttons may not be labeled
 skip-if = ["os == 'linux' && ccov"]  # Bug 1613515, the test consistently times out on Linux coverage builds.
 
 ["browser_locked.js"]