Bug 1854512 - Temporarily disable some Tier 2 a11y_checks for Core Document Object Model. r=smaug

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 skip the failing tests altogether and then handle each one of them individually.

This task is to skip with `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 1854513
- Explore capturing click events for a11y_checks: bug 1692110

Differential Revision: https://phabricator.services.mozilla.com/D188917

dom/tests/browser/browser.toml

@@ -34,6 +34,7 @@ https_first_disabled = true
 ["browser_bug1004814.js"]
 
 ["browser_bug1008941_dismissGeolocationHanger.js"]
+fail-if = ["a11y_checks"] # Bug 1854512 clicked #nav-bar.browser-toolbar may not be accessible
 tags = "geolocation"
 
 ["browser_bug1236512.js"]
@@ -71,6 +72,7 @@ support-files = ["prevent_return_key.html"]
 ["browser_focus_steal_from_chrome.js"]
 
 ["browser_focus_steal_from_chrome_during_mousedown.js"]
+fail-if = ["a11y_checks"] # Bug 1854512 clicked browser may not be accessible
 
 ["browser_form_associated_custom_elements_validity.js"]
 support-files = ["file_empty.html"]