Bug 1733631 - [ci] Stop keying 'mochitest-a11y' variants by-platform r=jmaher

Differential Revision: https://phabricator.services.mozilla.com/D128833

taskcluster/ci/test/mochitest.yml

@@ -126,12 +126,26 @@ mochitest-a11y:
     loopback-video: true
     e10s: false
     variants:
-        by-test-platform:
-            linux.*64/debug: ['fission', 'wayland', 'socketprocess_networking']
-            linux.*64-qr/debug: ['webrender-sw', 'fission', 'wayland', 'socketprocess_networking']
-            android-em-7.0-x86_64-qr/debug: ['webrender-sw', 'fission']
-            default: ['fission']
+        - fission
+        - socketprocess_networking
+        - wayland
+        - webrender-sw
     run-on-projects:
+        by-variant:
+            socketprocess_networking:
+                by-test-platform:
+                    linux.*64-qr/debug: built-projects
+                    default: []
+            wayland:
+                by-test-platform:
+                    linux.*64-qr/debug: built-projects
+                    default: []
+            webrender-sw:
+                by-test-platform:
+                    android-em-7.0-x86_64-qr/debug: built-projects
+                    linux.*64-qr/debug: built-projects
+                    default: []
+            default:
                 by-test-platform:
                     .*-tsan-qr/opt: ['trunk']
                     default: built-projects