test-utils: add cancellation support to soup_test_request_send

test-utils: add cancellation support to soup_test_request_send

The function gets a new parametter used to enable request cancellation for
both sync and async sessions. The cancellation could be performed by either
using the GCancellable or by directly cancelling the SoupMessage.

Also the GMainContext used to simulate sync operations with async ones will
now try to execute all its pending events before quiting the main loop.

https://bugzilla.gnome.org/show_bug.cgi?id=692310