Add modified file to bypass ugrid detection.

[pp-mo]

This was needed since introducing special handling of ugrid data in SciTools/iris#3684

Some regridding tests were using the file NetCDF/unstructured_grid/theta_nodal_xios.nc.
As it's a ugrid format file, the output from loading it has changed.
Those older tests need a version that loads without special-casing of ugrid data.
This PR adds a hacked version that does that.

[lbdreyer]

I'm struggling to see the differences with this PR.

I did an ncdump of the old and new file and then did a diff of the results but got no diff?

Sorry ignore all that! I hadn't properly checked out the new branch!

The diff looks like:

$ diff xios not_ugrid
1c1
< netcdf theta_nodal_xios {
---
> netcdf theta_nodal_not_ugrid {
10d9
< 		Mesh0:cf_role = "mesh_topology" ;
13a13
> 		Mesh0:NOT_cf_role = "mesh_topology" ;

[lbdreyer]

Merging this in as it seems a sensible change to add