Unit tests in test_bounding_box_tree.py wrongly rely on edge/facet indices

Some tests in test_bounding_box_tree.py wrongly rely on the computed indices for edges, facets, etc. DOLFIN provides no guarantees on the numbering of edges and faces, so a new mesh entity numbering algorithm causes these tests to fail.

The tests should use the vertices that make up an entity, or the position (midpoint) of an entity.