Add tests

[willirath]

(From https://github.com/ESM-VFC/xoak/pull/3#pullrequestreview-438701996.)

• [ ] test the XoakAccessor index property
• [ ] test non-transformed coordinate values (perhaps with other data than geo coordinates)
• [ ] test NN lookup tolerance
• [ ] check that the order of coordinate labels in the returned selection is preserved (I checked this locally simply by inspecting the results and it works)
• [ ] shallow test for the DataArray.xoak accessor

[benbovy]

Closed in #10, #30.