document ID/naming scheme

No need to get religious about this scheme IMO but tests will probably be easier to write and read if we have some conventions.

The convention used for element IDs so far is

'clickable-xyz' for links or buttons that the test script can click to trigger an action
'input-xyz' for input controls where the script can enter data
'list-xyz' for rows of data displayed
'section-xyz' for a certain part of a page

Beyond that, Stripes will always set following element IDs, using the module's npm name:

Navigation bar buttons: named clickable-<name>-module, for example

App pages: named <name>-module-display, for example

Then inside the checkout module itself the module developers have assigned following element ID's :

Using those identifiers, this test script will open the 'Check out' app and evaluate if an error is shown when an item is scanned before a patron: