[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.
