CKEditor repositions the dialog to attach an image after showing it. If
its position changes right when Capybara is trying to click the "Upload"
link, the click does not work properly.
I haven't found a way to check the dialog has stopped moving, so I've
chosen to force its position to the top.
Without this change, this test failed a lot on my machine when using
Rails 5.1, both with an "iframe ckeditor" and an "inline ckeditor".
However, it didn't fail when using Rails 5.0.