test-utils: set GSETTINGS_BACKEND=memory

To ensure we don't get unwanted proxy settings, and so we don't get
warnings if the memory backend is the only one available.
Fix soup_client_input_stream_close to not block

Closing a SoupClientInputStream for a message that hadn't been
completely read was trying to read to the end of the message first.
Fix it to just cancel the read instead.

Also fix a few tests that were implicitly assuming the old behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=695652