Fix test_figure_savefig_geotiff due to package updates

[seisman]

As shown in https://github.com/GenericMappingTools/pygmt/actions/runs/33465293617/job/99723779477?pr=4868, the test test_figure_savefig_geotiff fails with the following error messages:

            # TIFF
            with pytest.warns(expected_warning=NotGeoreferencedWarning) as record:
                with rioxarray.open_rasterio(fname) as xds:
                    pass
>           assert len(record) == 1
E           assert 4 == 1
E            +  where 4 = len(WarningsChecker(record=True))

It seems rioxarray raises NotGeoreferencedWarning four times rather than once. There are no new releases of rioxarray or rasterio, so it's unclear which package causes this failure.

This PR fixes the issue by changing len(record) == 1 to len(record) > 1.