test: Mark tst_qqmlmoduleplugin::incorrectPluginCase() as XFAIL

Mark incorrectPluginCase() as expected failure on OS X 10.8

Task-number: QTBUG-32652
Change-Id: I8fd2c0ceacabfc74defe84fc6538b268145c5110
Reviewed-by: Simo Fält <simo.falt@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>