DEV: Use actions matrix to parallelize ember cli tests (#14323)

.github/workflows/ember.yml

@@ -15,6 +15,11 @@ jobs:
     container: discourse/discourse_test:release
     timeout-minutes: 60
 
+    strategy:
+      fail-fast: false
+      matrix:
+        browser: ["Chrome", "Firefox", "Headless Firefox"]
+
     steps:
       - uses: actions/checkout@master
         with:
@@ -54,5 +59,5 @@ jobs:
 
       - name: Core QUnit
         working-directory: ./app/assets/javascripts/discourse
-        run: sudo -E -u discourse -H yarn ember test
+        run: sudo -E -u discourse -H yarn ember test --launch "${{ matrix.browser }}"
         timeout-minutes: 60