Expand tests to include more sample apps and version tests

[dazuma]

New tests include:

• Sample app tests for Sinatra 1, Rails 4, and Rails 5 in addition to the existing Rack test.
• Installation tests for all supported versions of Ruby installed from Google's package hosting, plus a test for a Ruby built from source.
• Several more assertions for key properties of the base image

[dazuma]

Details on the new tests:

• Some new utilities used in tests. These are generally docker commands that ensure any images/containers created during the command are cleaned up. Relevant files:
• appengine/test/test_helper.rb
• Updates to base image test. We check various things such as successful installation of node, bundler, rbenv, and default ruby. We also check that a few key environment variables are set. Relevant files:
• appengine/test/tc_base_image.rb
• Ruby versions test. This loops through a list of ruby versions, creates an image for each, and runs ruby --version to make sure it installed successfully. Relevant files:
• appengine/test/tc_ruby_versions.rb
• appengine/test/ruby_versions/Dockerfile
• Sample apps test update. Moved the apps into the sample_apps directory, and used the new utilities above to make the test code shorter. Relevant files:
• appengine/test/tc_sample_apps.rb
• The minimal_app sample app was moved into sample_apps and renamed rack_app. I updated it to use Rack 2.0, but it's otherwise unchanged.
• Created a Sinatra app test. Relevant files:
• appengine/test/sample_apps/sinatra1_app/Dockerfile
• appengine/test/sample_apps/sinatra1_app/myapp.rb
• Created a Rails 4 app test. Relevant files:
• appengine/test/sample_apps/rails4_app/Dockerfile
• appengine/test/sample_apps/rails4_app/app/controllers/home_controller.rb
• Created a Rails 5 app test. Relevant files:
• appengine/test/sample_apps/rails5_app/Dockerfile
• appengine/test/sample_apps/rails5_app/app/controllers/home_controller.rb