tst_QNetworkReply: Commentary fixes on test server workarounds.

Follow-up to commit 6fd205d5: document which version of danted shall
make one work-around redundant, document that another work-around is
still needed even with that v1.1.19; and remove a comment that
referred back to an XFAIL that commit 6fd205d5

 removed.

Change-Id: I270b662528127c82184bff20b3cecea4f0c41b41
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>