Load only tested plugins when invoked with -test

By default, a clean settings path is used for test environment.

All the default plugins are loaded, although they're not needed.

This change significantly improves loading time for tests.

Change-Id: I24254f3e538e3f0e6d233d0989738dc1ce238209
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>