tests: fix tst_qdeclarativemoduleplugin::incorrectPluginCase()

It gives different error message based on the case sensitivity of
the file system on Mac.

Task-number: QTBUG-32646
Change-Id: I8b5986a4bc8886924e678692c6679e571a11dd33
Reviewed-by: Alan Alpert (Personal) <416365416c@gmail.com>