Deprecation warnings in the tests

[zmoon]

I noticed some deprecation warnings when running the tests that could be addressed, related to

• setting the background color of a GeoAxes
• xarray argmin behavior.

xmovie/test/test_core.py: 144 warnings
  /mnt/c/Users/zmoon/git/xmovie/xmovie/presets.py:168: DeprecationWarning: The background_patch property is deprecated. Use GeoAxes.patch instead.
    ax.background_patch.set_facecolor(bgcolor)

xmovie/test/test_core.py: 48 warnings
  /home/zmoon/miniconda3/envs/xmovie-test/lib/python3.10/site-packages/xarray/core/dataarray.py:4407: DeprecationWarning: Behaviour of argmin/argmax with neither dim nor axis argument will change to return a dict of indices of each dimension. To get a single, flat index, please use np.argmin(da.data) or np.argmax(da.data) instead of da.argmin() or da.argmax().
    result = self.variable.argmin(dim, axis, keep_attrs, skipna)