`test_array_object.py::test_getitem_masking` relies on shape assertion

[tomwhite]

test_array_object.py::test_getitem_masking fails in Dask due to a assert_shape call on a masked array. This should be hidden behind the --disable-data-dependent-shapes flag.

See https://github.com/tomwhite/dask/runs/5306535598?check_suite_focus=true

[tomwhite]

Also dask/dask#8750