The change 47c72c2 tried to address the
intermittendly failing test, but I think made the mistake of
"hardcoding" the file we assume to be removing from the attachment list.

The problem however is, that the list of files we're adding as
attachment varies with the environment. While the `screenshot.png` is
probably the attachment in most cases, it isn't the attachment
in **all** cases. The assertion that the next screen does not show a
file which we never added is always true.

Instead, firstly, this patch asserts that the last filename is actually
listed as an attachment. Secondly, we assert that when the attachment is
removed the last file from before is gone. Furthermore we also check
whether the item count has changed.

Fixes: #327

This uses what we've learned in the previous change and applies it to
the remaining tests using a "Not" condition without asserting the
presence of the element in the first place. This only applies to
asserting that something was present in the previous screen and is now
gone in the subsequent screen.