gha: add merge_group trigger

See https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue#triggering-merge-group-checks-with-github-actions

.github/workflows/tests.yaml

@@ -4,6 +4,7 @@ on:
   push:
     branches: [main]
   pull_request:
+  merge_group:
   workflow_dispatch:
   schedule:
     - cron: "20 4 * * 2" # once a week