tests(fix): wait for XHR to set the display on the error div

@vladikoff - I realize that waitForVisibleByCssSelector is deprecated in leadfoot, so what is the correct way (if it exists) to handle this case: an element exists in the DOM but is only displayed after an XHR returns. 

The way it is right now, `.findByCssSelector('#stage .error').isDisplayed()` wins the race over `.findById('submit-btn').click()` and the test incorrectly fails when run on a remote server.

If there's a better pattern to use, let me know and I'll update the PR. Thanks.