Testing: Rename srcdump-loginback.at (from srcdump.at)

This actually tests a rather artificial use case, where the user
bookmarks the url, then logs in normally, and then uses the saved url
(or achieves the equivalent by using the back button).

It should have a name that better reflects this.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>