Bug 1854514 - Temporarily disable some Tier 2 a11y_checks for New Tab Page. r=thecount

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 1854515
- Explore capturing click events for a11y_checks: bug 1692110

Differential Revision: https://phabricator.services.mozilla.com/D188918

browser/components/newtab/test/browser/browser.toml

@@ -38,6 +38,7 @@ skip-if = ["os == 'linux' && bits == 64 && debug"] # Bug 1784548
 ["browser_aboutwelcome_glean.js"]
 
 ["browser_aboutwelcome_import.js"]
+fail-if = ["a11y_checks"] # Bug 1854514 clicked primary button may not be focusable
 
 ["browser_aboutwelcome_mobile_downloads.js"]
 
@@ -82,6 +83,7 @@ skip-if = [
 
 ["browser_asrouter_cfr.js"]
 https_first_disabled = true
+skip-if = ["a11y_checks"] # Bug 1857056 for causing crashes
 
 ["browser_asrouter_experimentsAPILoader.js"]