[rset] refactoring of _build_entity method define a method for entity instantiation and storage in cache (related to #9942503)

[entity] comparing two entities returns NotImplemented Fixes a test when using some versions of unittest2, where assertItemsEqual tries a sort and expects TypeError, which it doesn't get if we directly raise NotImplementedError. The NotImplemented value is treated specially by the interpreter: https://docs.python.org/3.4/library/constants.html#NotImplemented

[test] replace one use of lgc.testlib.unittest_main with unittest.main