rspec.rake

RSpec::Core::RakeTask.module_eval do
  def pattern
    dir = EngineName.root # replace with you the name of your engine
    extras = []
    if File.directory?( dir )
      extras << File.join( dir, 'spec', '**', '*_spec.rb' ).to_s
    end
    [@pattern] | extras
  end
end

Thanks for posting this. This helped.