lttng: Make ControlFlowViewTest more robust

lttng: Make ControlFlowViewTest more robust

Added a condition to wait on. Added the concept of the view being dirty
so that we know if the view is ready or not. This required some rework
of the zoom thread methods to make it less error prone in keeping the
dirty flag consistent.

Change-Id: Ib73d84363e90aae1ca046c101f2086e3840b5bc2
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/62154
Reviewed-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>