[test] Prefer unittest.main

[test] py.test related fix We ought not import a concrete test case from a sibling module, else it ends up partially initialized at some point and later raise assertion error in setUpClass.

[dataimport] add a bit of extra-documentation on the massive store about prepare_insert_relation limitation on inlined relation and lack of full-text indexation.

[dataimport] no need to call .keys()