CppTools: Tests: Clean up cppmodelmanager_test.cpp

* First construct a ProjectPart completely, then append it to the
  ProjectInfo. Doing it the other way around will lead to
  inconsistencies since some attributes of ProjectInfo are
  calculated/updated when a ProjectPart is appended. This has no effect
  on the current tests.

* Remove created *.user file when test finishes.

* Add comments describing what the tests do.

* Rename test methods to some more desribtive names.

Change-Id: I5eeed05d0aef19f8094ff488dedcced74f0814f8
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>