Bug 1854522 - Temporarily disable one Tier 2 a11y_checks for Core Necko. r=necko-reviewers,valentin

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 bug 1854523
- Explore capturing click events for a11y_checks: bug 1692110

Differential Revision: https://phabricator.services.mozilla.com/D188921

netwerk/test/browser/browser.toml

@@ -131,6 +131,7 @@ skip-if = [
 ["browser_bug1535877.js"]
 
 ["browser_bug1629307.js"]
+fail-if = ["a11y_checks"] # Bug 1854523 clicked button may not be focusable
 
 ["browser_child_resource.js"]
 run-if = ["crashreporter"]