fix race condition in order details spec

:bomb: Should fix sporadic failures on Travis CI. /cc @jdutil

The issue was our tests passing prematurely, and then causing errors as DatabaseCleaner ran.

This behaved strangely because this portion of the admin displays changes by using  `window.location.reload()`, which `wait_for_ajax` doesn't wait for (after all, it isn't AJAX). This improves the tests by waiting for the input fields to disappear instead.