C++: Tests: Generalize for using different test data dirs

...for the modelmanager tests.

Change-Id: I91f163df1ed958a3915ce263b12e3e30cda0b78f
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
C++: Fix 'already seen files' when indexing

Resetting the environment (after each *.cpp file) did not clear the
already seen files (m_included). Because of that the succeeding header
files were not parsed correctly - the environments of the mistakenly
already seen header files were not merged in.

Note that this change slow downs the parsing/indexing of files to its
original speed, as it was before the problematic commit (and it is in
2.7):

    commit 82e347095c4bdb7a6419d28798639e60591e3992
    C++: Untangle include file resolving from loading.

Task-number: QTCREATORBUG-9205
Change-Id: Iea57b7c59ea04a3c8843fd1291f4c375382958fc
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>