Fix dumpdir fixture

[connorjward]

Previously the dumpdir fixture would rely on the mesh fixture. This lead to some confusing behaviour where

$ pytest tests/firedrake/output/test_pvd_output.py::test_bad_cell --co

was giving the very confusing result of

collected 8 items                                                                                                     

<Dir firedrake>
  <Dir tests>
    <Dir firedrake>
      <Dir output>
        <Module test_pvd_output.py>
          <Function test_bad_cell[interval]>
          <Function test_bad_cell[square[tri]]>
          <Function test_bad_cell[square[quad]]>
          <Function test_bad_cell[box[tet]]>
          <Function test_bad_cell[box[quad x interval]]>
          <Function test_bad_cell[box[hex]]>
          <Function test_bad_cell[sphere[tri]]>
          <Function test_bad_cell[sphere[quad]]>

despite test_bad_cell not needing the mesh fixture.