Checkout PR merge commit for CI tests (#468)

Author: albertvillanova

.github/workflows/tests.yaml

@@ -18,7 +18,6 @@ jobs:
        uses: actions/checkout@v3
        with:
         lfs: 'true'
-        ref: ${{ github.event.pull_request.head.sha }} # we want to test against our branch not against a merge commit
      - name: Setup Python environment
        uses: actions/setup-python@v4
        with: