[entity] Add a cw_related_rqlst method returning the RQL select node of cw_related method This is useful if one wants to modify the query before execution. A bit a PEP8 style fix in tests along the way. Related to #12306543.

[devtools] set .config in setUpClass instead of using a cache in a classproperty This is much clearer and avoid problem with py.test introspection that triggered creation and caching of the configuration before actual test execution. This caused pb in unittest_postgres.PostgresStatementTimeoutTC where configuration was built before module's setup, hence source configuration was missing information about where to find the pg cluster.

[devtools] Simplify test configuration's init Specify module's __file__ instead of apphome. __file__ is saved on the class as it will be used later (eg to find the proper pg cluster without relying on shared dictionnary modifications). This will require some tests update, but it should be less common in cubes than in cubicweb itself.