Revert "Fix mix of new[] / malloc in QTest::toHexRepresentation"

This reverts commit e8f73d656cf13d440a3a83980e05c6b117489e40. 
The original was correct: qtestcase.cpp:282 and 283 use delete[].

Change-Id: I668e17f500edcf9f1406553a160f6f5518dee148
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>