Code review comment for lp:~alan-griffiths/mir/drag-and-drop

OK, that's a better failure mode.

I'm hoping the follow-up fixes the race that causes this, in which case I'll merge the MPs and resubmit.

11:34:50 9: [ RUN ] DragAndDrop.during_drag_when_user_moves_mouse_client_receives_handle
11:34:50 9: [2017-03-14 11:34:50.894056] mirserver: Starting
11:34:50 9: [2017-03-14 11:34:50.897709] mirserver: Selected driver: mir:stub-graphics (version 0.27.0)
11:34:50 9: [2017-03-14 11:34:50.919402] mirserver: Using software cursor
11:34:51 9: [2017-03-14 11:34:51.080386] mirserver: Selected input driver: mir:stub-input (version: 0.27.0)
11:34:51 9: [2017-03-14 11:34:51.096963] mirserver: Mir version 0.27.0
11:34:51 9: [2017-03-14 11:34:51.103497] mirserver: Initial display configuration:
11:34:51 9: [2017-03-14 11:34:51.103792] mirserver: Output 1: VGA connected, used
11:34:51 9: [2017-03-14 11:34:51.104124] mirserver: Physical size 0.0" 0x0mm
11:34:51 9: [2017-03-14 11:34:51.104267] mirserver: Power is on
11:34:51 9: [2017-03-14 11:34:51.104432] mirserver: Current mode 800x600 60.00Hz
11:34:51 9: [2017-03-14 11:34:51.104592] mirserver: Preferred mode 800x600 60.00Hz
11:34:51 9: [2017-03-14 11:34:51.104726] mirserver: Orientation normal
11:34:51 9: [2017-03-14 11:34:51.104871] mirserver: Logical size 800x600
11:34:51 9: [2017-03-14 11:34:51.105010] mirserver: Logical position +0+0
11:34:51 9: [2017-03-14 11:34:51.105154] mirserver: Scaling factor: 1.00
11:36:21 9: /<<BUILDDIR>>/mir-0.26.1+17.04.20170209.1+zesty4306bzr4109/tests/acceptance-tests/drag_and_drop.cpp:191: Failure
11:36:21 9: Value of: have_cookie.wait_for(receive_event_timeout)
11:36:21 9: Expected: is equal to true
11:36:21 9: Actual: false (of type bool)
11:36:21 9: /<<BUILDDIR>>/mir-0.26.1+17.04.20170209.1+zesty4306bzr4109/tests/acceptance-tests/drag_and_drop.cpp:286: Failure
11:36:21 9: Value of: cookie.get()
11:36:21 9: Expected: isn't NULL
11:36:21 9: Actual: NULL (of type MirCookie const*)
11:36:21 9: [2017-03-14 11:36:21.680766] mirserver: Stopping
11:36:21 9: [ FAILED ] DragAndDrop.during_drag_when_user_moves_mouse_client_receives_handle (90835 ms)