test: skip SAX external entities test in older libxml2

Before GNOME/libxml2@eddfbc38 the `#characters` event was duplicated
in this test case. I'm not going to work around it in Nokogiri.

test/xml/sax/test_parser.rb

@@ -634,6 +634,8 @@ class TestCase
             end
 
             it "resolves local external entities when replace_entities is true" do
+              skip if Nokogiri.uses_libxml?("< 2.9.11") # gnome/libxml2@eddfbc38
+
               Tempfile.create do |io|
                 io.write("local-contents")
                 io.close