gtk_test_init() ends up calling gtk_init() which tries opening
the default display. CI jobs sometimes seem to trip over this
even when run through xvfb-run. Therefore we use g_test_init()
instead.
Using rescan allows use to give a priority to search paths. So this way
plugins in `CALLS_PLUGIN_DIR` take precedence over plugins we ship.
This also makes sure that the plugin test searches in the same location
as CallsManager.
Since we will introduce another type of plugin for the policy engine
we want to have each plugin type in separate directories.
We also have to adjust:
- plugin search directories
- po file location
- update paths for calls-doc target
