gio/tests/task: fix a race condition in test_run_in_thread()

gio/tests/task: fix a race condition in test_run_in_thread()

When running a task in a thread, GTask may still be internally holding
a ref on the task in that thread even after the callback is called in
the original thread (depending on thread scheduling). Fix the test to
handle that by using a weak notify that signals a GCond, and wait for
that GCond from the main thread. (And add a corresponding check to
test_return_on_cancel().)

https://bugzilla.gnome.org/show_bug.cgi?id=705152