Fix memleaks in duchain unittests

Summary:
The TypeSystem::create test was changed as now the destruction is also taken into account for QBENCHMARK

The TopDUContext dtor is protected, however you can execute
delete TopDUContext; but it will NOT call the protected TopDUContext dtor but the public DUContext dtor.

Test Plan:

• compile kdevelop unitest with ASAN&LSAN
• execute test_duchain

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D16458