[config] Test loading of ccplugin modules to ensure ccplugin modules of cube as package aren't loaded twice, once as `cubicweb_<cube>.ccplugin`, once as `cubes.<cube>.ccplugin`. This test used to fail until fe995d56c949 has been backout. It requires calling `cleanup_sys_module` in `CubicWebConfigurationTC.tearDown` to remove any cube's submodule loaded during tests, so content of `sys.modules` may be tested.