ci: add MongoDB auth credentials to connection URI

- Add testuser:testpass to MONGO_ATLAS_URI for CI environment
- Fixes 'command createIndexes requires authentication' errors
- Tests that don't use mock fixtures need real MongoDB connection

Author: bchou9

.github/workflows/ci-tests.yml

@@ -88,7 +88,7 @@ jobs:
       - name: Create .env file
         run: |
           cd backend
-          printf "MONGO_ATLAS_URI=mongodb://localhost:27017/\n\
+          printf "MONGO_ATLAS_URI=mongodb://testuser:testpass@localhost:27017/?authSource=admin\n\
             RESILIENTDB_GRAPHQL_URI=https://cloud.resilientdb.com/graphql\n\
             RES_DB_BASE_URI=https://crow.resilientdb.com\n\
             SIGNER_PUBLIC_KEY=%s\n\