gdbus: fix deadlock on message cancel/timeout

The gdbus GTask port introduced a deadlock because some code had been
using g_simple_async_result_complete_in_idle() to ensure that the
callback didn't run until after a mutex was unlocked, but in the gtask
version, the callback was being run immediately. Fix it to drop the
mutex before calling g_task_return*(). Also, tweak
tests/gdbus-connection to test this.

https://bugzilla.gnome.org/show_bug.cgi?id=747349