Let hg Ignore py.test's .cache directory And drop rule for .svn along the way.

[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.