In the writev() tests, the handling of cancellation is tested. However,
the GCancellable was cancelled after the writev_async() call was
started. Depending on the implementation of the writev() vfunc, the
operation could be done in a thread or in callbacks on the current
thread’s main loop. If done in a separate thread, there’s a chance that
enough of the write could happen before cancellation reaches that thread
that the overall operation returns success with a short write.

That would cause the test to fail, sometimes.

Avoid that by cancelling the GCancellable before starting the writev()
operation.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Reviewed-by: nobody