test.yml using wrong ref

[bj00rn]

I believe the current test.yml workflow is using the wrong ref to test PRs. The commit actually being tested in the workflow is base branch ref since the GITHUB_REF for pull_request_target PR base branch.

The correct syntax is as follows, to check out PR head sha.

 - uses: actions/checkout@v3
     with:
          ref: ${{ github.event.pull_request.head.sha }}
      - name: Rerun on comment