fix race condition in order details spec

:bomb: [Should fix sporadic failures on Travis CI](https://travis-ci.org/jhawthorn/spree/builds/20383160) (and elsewhere) /cc @jdutil 

The issue was that our tests passed prematurely, then unfinished requests caused errors after DatabaseCleaner had run.

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 changes the specs to wait for the input fields to disappear instead.

*Fixed forever ™*