CppEditor: tests for "Include Hierarchy"

Very simple tests for include hierarchy model:
* test if checked file includes one file
* test if checked file is included by one file
* test if checked file includes one file and is included by one file

Change-Id: I3688f9ddb86a569f780fc156bdfcf6b81a9fb0c1
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>