Fix tst_QCompleter::directoryModel() on OS X

By not assuming that we have the '/Developer' directory at the root
of the file system. 'Users' is less likely to be removed/deprecated.

Change-Id: I659bdb67cfb1ed2f73bc643ba4afe1f1f89d5bc5
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>