app.rb

require "rubygems"
require "rack"
$:.unshift File.dirname(__FILE__) + "/lib"
require "sinatra/base"

class Sinatra::Application
  get "/" do
    "Hello World"
  end
end

app_test.rb

require "test/unit"
require "rack_session"
require "app"

class AppTest < Test::Unit::TestCase
  def setup
    @app = RackSession.new(Sinatra::Application)
  end

  def test_it_works
    @app.get "/"
    assert_equal 200          , @app.response.status
    assert_equal "Hello World", @app.response.body
  end
end

output.txt

~/code/sinatra (hoboken-test) % ruby app_test.rb 
Loaded suite app_test
Started
.
Finished in 0.006002 seconds.

1 tests, 2 assertions, 0 failures, 0 errors

rack_session.rb

class RackSession
  attr_reader :response, :request, :app

  def initialize(app)
    @app = app
  end

  def get(path, data={}, headers={})
    do_request(:get, path, data, headers)
  end

  def post(path, data={}, headers={})
    do_request(:post, path, data, headers)
  end

  def put(path, data={}, headers={})
    do_request(:put, path, data, headers)
  end

  def delete(path, data={}, headers={})
    do_request(:delete, path, data, headers)
  end

  private
    def do_request(verb, path, data={}, headers={})
      if data.is_a?(String)
        headers.update(:input => data)
        data = {}
      end

      @request = Rack::MockRequest.new(app)
    end

    def uri(path, data={})
      uri = URI(path)
      uri.query = data.map {|k,v| "#{k}=#{v}" }.join("&")
      uri.to_s
    end
end