Refactor test parameterization for some IO tests

[connorjward]

This means that we see things like

          <Function test_io_function_base[triangle_small-P-1]>
          <Function test_io_function_base[triangle_small-P-6]>
          <Function test_io_function_base[triangle_small-DP-0]>

instead of

          <Function test_io_function_base[cell_family_degree0]>
          <Function test_io_function_base[cell_family_degree1]>
          <Function test_io_function_base[cell_family_degree2]>