CppTools: Fix test_modelmanager_defines_per_project

The example project parts had no project file set and thus the "right"
project part could not be determined in cppeditor.cpp:718.

Change-Id: I088571e9d3c3c8790670bbdd8e68190835a85ad5
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>