Backing out enabling of test_plugin_position.xhtml while we back out the fix for that test

layout/generic/test/Makefile.in

@@ -78,9 +78,10 @@ _TEST_FILES = \
 		test_movement_by_characters.html \
 		test_movement_by_words.html \
 		test_plugin_clipping.xhtml \
-		test_plugin_position.xhtml \
 		test_selection_underline.html \
 		$(NULL)
+# disabled temporarily
+# 		test_plugin_position.xhtml \
 
 libs:: $(_TEST_FILES)
 	$(INSTALL) $(foreach f,$^,"$f") $(DEPTH)/_tests/testing/mochitest/tests/$(relativesrcdir)