Incompatibility with latest `desiutils` versions

Since v3.5.0, `desiutils` has removed the `desiutil.setup.DesiTest` class ([changes.rst](https://github.com/desihub/desiutil/blob/main/doc/changes.rst#350-2025-01-13)).

If `desiutils` is installed, this causes the LSS installation to fail due to the call to `DesiTest` line 83 of `setup.py`. Removing the line makes the install process work but I suppose it might break the testing somewhere else.

For example the current cosmodesienv-test switches to a more recent version of `desiutils` so this is likely to become an issue for some people when the main is updated.