Add tests for 2d coords contouring

[pp-mo]

At present (https://github.com/SciTools/iris/tree/5923aaa8e725831a1575678fd93c938bfad36e95), we test :

• various tests for 2d coord handling functions in tests.unit.plot : test__get_plot_defn[_custom_coords_picked], test__check_bounds_contiguity_and_mask
• integration tests for pcolormesh in tests.integration.plot : test_plot_2d_coords.

We ought to also have an integration test proving 2d coords operation in contour/contourf
Probably also belongs in test_plot_2d_coords