An invalid mime magic definition could lead to an assert. Replaced with
a qWarning. Move all checking to the QMimeMagicRule constructor, and do
keep invalid rules since they are need to parse child rules.

Unit test added, with QTest::ignoreMessage when using the XML backend
(there's no warning from update-mime-database when using the cache).
Also make it easier to add more shared mime info files for tests.

Task-number: QTBUG-44319
Done-with: Eike Ziller <eike.ziller@theqtcompany.com>
Change-Id: Ie39a160a106b650cdcee88778fa7eff9e932a988
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>