test_downstream: only transfer info from upstream req.txt if it exists

  cd "$proj_name"

  echo "*** $proj_name version: $(git rev-parse --short HEAD)"

  if test -f ../requirements.txt; then

    transfer_requirements_git_urls ../requirements.txt ./requirements.txt

  fi

  edit_requirements_txt_for_downstream_in_subdir

  # Avoid slow or complicated tests in downstream projects