plugin: plugin_load() must return a ref even if it was loaded already

plugin: plugin_load() must return a ref even if it was loaded already

Fix refcounting issue when plugin was loaded already.
gst_plugin_load() is supposed to return a ref, so it
must always return a ref.

This also fixes the gstplugin unit test on windows where
fork is not available and where test_load_coreelements()
would unref a plugin ref it didn't get and then mess up
the internal registry plugin list state for the next test,
in case where the test registry does not exist yet.