Use tests-passed branch of discourse/discourse in tests action

.github/workflows/plugin-tests.yml

@@ -52,7 +52,7 @@ jobs:
       - uses: actions/checkout@v2
         with:
           repository: discourse/discourse
-          ref: "${{ (github.base_ref || github.ref) }}"
+          ref: "tests-passed"
           fetch-depth: 1
 
       - name: Fetch Repo Name