lttng: Fix intermittent failure in MemoryUsageViewTest

lttng: Fix intermittent failure in MemoryUsageViewTest

It's possible that the series are not there yet after the jobs running.
This is because of an asyncExec in the view code.
To reproduce the issue, in TmfCommonXLineChartViewer.updateDisplay,
change the asyncExec to a timerExec(5000, ...)

We can use a waitUntil to wait until the series are there.

Change-Id: Ic70d3adbec88578c155b53a4f42e51c568180511
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/66865
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>