Bug 1602863 - run mochitest-a11y with ubuntu1804 docker image r=jmaher

Changes:

Run `mochitest-a11y` on ubuntu1804 image.

On the `ubuntu1804` image, bug 1429575 is a relatively frequent occurrence.

Differential Revision: https://phabricator.services.mozilla.com/D59220

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

taskcluster/ci/test/test-sets.yml

@@ -344,7 +344,6 @@ linux-common-tests:
     - jittest
     - marionette
     - mochitest
-    - mochitest-a11y
     - mochitest-browser-chrome
     - mochitest-chrome
     - mochitest-devtools-chrome
@@ -364,6 +363,7 @@ linux1804-tests:
     - crashtest
     - gtest
     - jsreftest
+    - mochitest-a11y
     - mochitest-gpu
     - mochitest-media
     - mochitest-webgpu
@@ -375,6 +375,7 @@ linux1804-qr-tests:
     - crashtest
     - gtest
     - jsreftest
+    - mochitest-a11y
     - mochitest-gpu
     - mochitest-media
     - mochitest-webgpu
@@ -384,7 +385,6 @@ linux1804-qr-tests:
 linux-qr-tests:
     - jittest
     - mochitest
-    - mochitest-a11y
     - mochitest-chrome
     - mochitest-webgl1-core
     - mochitest-webgl2-core