soup_session_cancel_message: fix up, especially in sync sessions

soup_session_cancel_message: fix up, especially in sync sessions

Cancelling a message from another thread had some race conditions that
could sometimes cause crashes. Fix things up a bit by using
GCancellable to interrupt the I/O, rather than calling
soup_message_io_finished() directly.

Also added a test for this case to tests/misc-test, although
unfortunately due to the raciness of the bug, it only failed
sporadically even before the fix (but seems to fail never now).

https://bugzilla.gnome.org/show_bug.cgi?id=637741