testcases: Constify missed msg variables.

This finishes the fixes for change in parse_opts(), I've missed a few in:

commit 0b9589f3f9c0345b29cfcf7da5a1253c708303eb
Author: Cyril Hrubis <chrubis@suse.cz>
Date:   Tue May 27 17:40:33 2014 +0200

    testcases: Constify char *msg variable.

    The parse_opts() now returns const char * instead of char * this commit
    fixes all warnings caused by the change.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>