Use lazily_find_element in order_details_spec rather than sleep
common:test_app rake task generator lookup changes

now expects to find an extension's install_generator in the install directory
camelizes the lib_name instead of classifying so that correct generator class names are created for plural nouns (e.g. spree_simple_sales)

Related to #2863
DummyGenerator changes to improve dummy app generation for extensions

created a new extension:test_app rake task to handle dummy app generation for extensions
the extension template Rakefile calls this new task
DummyGenerator now injects correct requirements into the dummy app's config/application.rb file when called from the new rake task

Related to #2863
Added --auto-run-migrations option to install generators create for extensions.

When the common:test_app task is invoked for an extension, it now runs the extension's InstallGenerator with the --auto-run-migrations option so the dummy app is generated in a single step without the need for user input.

Related to #2863