tests: add plugin loading whitelist to test environment

Only want to load core/base/good plugins here.

Fixes #619717.

tests/check/Makefile.am

@@ -10,6 +10,7 @@ TESTS_ENVIRONMENT = \
 	$(REGISTRY_ENVIRONMENT)					\
 	GST_PLUGIN_SYSTEM_PATH=					\
 	GST_PLUGIN_PATH=$(top_builddir)/gst:$(top_builddir)/ext:$(top_builddir)/sys:$(GSTPB_PLUGINS_DIR):$(GST_PLUGINS_DIR)			\
+	GST_PLUGIN_LOADING_WHITELIST="gstreamer@$(GST_PLUGINS_DIR):gst-plugins-base@$(GSTPB_PLUGINS_DIR):gst-plugins-good@$(top_builddir)"	\
 	STATE_IGNORE_ELEMENTS="aasink autoaudiosrc autoaudiosink autovideosrc autovideosink \
 		cacasink cairotextoverlay gconfaudiosrc gconfvideosrc gconfaudiosink gconfvideosink \
 		halaudiosrc halaudiosink v4l2src osssrc osssink pulsesink pulsesrc pulsemixer \