Print the used seed as a comment

Sometimes when your tests have an ordering dependency (i.e. examples fail intermittently depending on order) you need to know the seed which was used for randomizing the order to reproduce that failure. This change will result in a top-level comment containing the seed. I did not implement it as an attribute to avoid clashes with the XML schema.

Note: As I understand from https://github.com/rspec/rspec-core/blob/master/lib/rspec/core/formatters.rb , you should overwrite the method `seed(number)` to produce that output, but this method never gets called.