Skip qplugin auto test for static configurations

All test cases use QPluginLoader which will not work for static builds of
Qt.

Change-Id: I7dcddcd5213681bd3ae4cd85e49ee0bb5748f687
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

tests/auto/corelib/plugin/plugin.pro

@@ -11,5 +11,6 @@ qtConfig(library): SUBDIRS += \
 contains(CONFIG, static) {
     message(Disabling tests requiring shared build of Qt)
     SUBDIRS -= qfactoryloader \
+               qplugin \
                qpluginloader
 }