irb

blake:carealot$ irb -rubygems -r myapp.rb -r sinatra/test

irb(main):001:0> app = Sinatra::TestHarness.new(Sinatra::Application)
=> #<Sinatra::TestHarness:0x10c3d74 @app=Sinatra::Application>

irb(main):002:0> app.get '/'
=> #<Rack::MockResponse:0x10ae780 @original_headers={"Content-Type"=>"text/html", "Content-Length"=>"7"}, @status=200, @errors="", @headers={"Content-Type"=>"text/html", "Content-Length"=>"7"}, @body="testing">

irb(main):003:0> app.body
=> "testing"

myapp.rb

require 'sinatra'

get '/' do
  'testing'
end