org.eclipse.e4.ui.tests.application.EPartServiceTest

In HeadlessContextPresentationEngine it is incorrectly assumed that a
parent must have a widget, this is only true when the full parent/host
chain is to be rendered.

Change-Id: Ieaa0daf21779f3c1e6535b948de6d3f8b95ab784
Signed-off-by: Rolf Theunissen <rolf.theunissen@gmail.com>