C++: Ensure test code is free of diagnostic messages in tst_checksymbols.cpp

Change-Id: I11b3caf354d57f08268cca7bc2944ca7ae386bfb
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
CppEditor: Use the project part chosen with CppPreProcessorDialog

...otherwise the CppPreProcessorDialog is only semi-useful.

Change-Id: I6b80967b05e7c9a51cd78ae064a3d78170948e32
Reviewed-by: David Schulz <david.schulz@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>