define 'ucx' pytest marker (#2281)

Looking through logs on #2279, I noticed this warning:

```text
test/test_comms.py:267
  /__w/raft/raft/python/raft-dask/raft_dask/test/test_comms.py:267: PytestUnknownMarkWarning: Unknown pytest.mark.ucx - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
    @pytest.mark.ucx
```

([build link](https://github.com/rapidsai/raft/actions/runs/8850602382/job/24314273393?pr=2279#step:7:780))

This resolves it.

### How I tested this

Looked for all such cases like this:

```shell
git grep -E 'mark.*ucx'
```

Hopefully we'll see this log disappear from the `conda-python-tests` CI logs on this PR.

Authors:
  - James Lamb (https://github.com/jameslamb)

Approvers:
  - Bradley Dice (https://github.com/bdice)
  - Dante Gama Dessavre (https://github.com/dantegd)
  - Ray Douglass (https://github.com/raydouglass)

URL: #2281

python/raft-dask/pytest.ini

@@ -6,3 +6,4 @@ markers =
   mg: marks a test as multi-GPU
   memleak: marks a test as a memory leak test
   nccl: marks a test as using NCCL
+  ucx: marks a test as using ucx-py