tests: tst_qdeclarativelanguage: fix filesystem case sensitivity

importIncorrectCase() and errors(incorrectCase) give different error
messages depending on the case sensitivity of the file system on OS X.
Those are in turn different than error messages on other operating
systems with case-sensitive filesystems.

This patch is very similar to the one that was done in qtdeclarative:
2ebf4ba4710a01a038d05cfea741e5538bb4567f for QTBUG-37622

Change-Id: Ib13d717712cf88b476ca7edd8aababd7f5456c50
Task-number: QTBUG-41961
Reviewed-by: Liang Qi <liang.qi@digia.com>