Bug 1546532 - enable mochitest-a11y for windows10-aarch64 r=jmaher,gbrown

Differential Revision: https://phabricator.services.mozilla.com/D28579

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/test-sets.yml

@@ -259,6 +259,7 @@ windows-aarch64-tests:
     - marionette
     - marionette-headless
     - mochitest
+    - mochitest-a11y
     - mochitest-browser-chrome
     - mochitest-chrome
     - mochitest-clipboard