Fix a bug in qsort

In commit (35334ca1) qsort was fixed but unfortunately a small
typo was introduced. Due to that typo the qsort made its job wrong.

Fix the problem and add unit test for qsort.

Unfortunately the test right from the issue runs extremely long,
so it should go to long-tests.

Closes #7605

NO_DOC=bugfix

(cherry picked from commit e1d96170)