Bug 19049: [Follow-up] Make sure that GetPlugins finds the plugin

Bug 19049: [Follow-up] Make sure that GetPlugins finds the plugin

We have to mock_config the pluginsdir before Plugins is loaded, and
we should pass an absolute path (not a relative one).
If you did not install the test to_marc plugin, this test would fail.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Verified patch is compatible with original KitchenSink to_marc plugin

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>