Exclude a failing method spec for Digest

Author: andrykonchin

spec/tags/truffle/methods_tags.txt

@@ -101,3 +101,4 @@ fails:Public methods on Method should include dup
 fails:Public methods on String should include dup
 fails:Public methods on UnboundMethod should include dup
 fails:Public methods on StringIO should include pread
+fails:Public methods on Digest::Base.singleton_class should not include inherited