Testlib: Use QString for messages in QAbstractTestLogger::addMessage()

Task-number: QTBUG-35743

[ChangeLog][QtTest][Windows] Use correct UTF-8 encoding for XML test results on platforms with different console encoding.
Change-Id: Ice9d03192098f931e5dac358928e0c4421ab715e
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>