Demonstate issue with wonky constants on 1.9.2

I'm expecting this to fail, the reason why the spec is a new file is that 1.9.2's issue with wonky constants seems to stem from the first `instance_exec` not  the last, so having this inside an existing example group 
doesn't cause the bug to occur.

See #1697 

/cc @myronmarston
I think a better way to do this is:

``` ruby
def self.base_name_for(group)
  # existing logic
end

if RUBY_VERSION == '1.9.2'
  alias _base_name_for base_name_for
  private_class_method :_base_name_for
  def base_name_for(group)
    _base_name_for(group) + '_'
  end
end
```

That way, there's no `_base_name_for` method on other versions since it's not needed.