tst_QSet: check which of several equal elements is inserted

Add a test that checks that QSet keeps the first of the elements
that have equal value. This is documented, but inconsistent with
values in a QHash, which keeps the last element with equal key.

Document this as a test. That way, we'll be informed when the
behavior changes (e.g. by a port to std::unordered_set).

Change-Id: I4ca1718bb86599b925b3ccd13b0856917cd4ce67
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>