ci: add sign-commits flag to test suite comment

Author: peter-evans

.github/workflows/ci.yml

@@ -108,6 +108,7 @@ jobs:
             Full test suite slash command (repository admin only)
             ```
             /test repository=${{ github.event.pull_request.head.repo.full_name }} ref=${{ github.event.pull_request.head.ref }} build=true
+            /test repository=${{ github.event.pull_request.head.repo.full_name }} ref=${{ github.event.pull_request.head.ref }} build=true sign-commits=true
             ```
 
   package: