tests/qtest: fix discarded const qualifier warning

Modern compilers warn that the result of strstr() may discard
const qualifiers when assigned to a non-const pointer.

Make 'found' a const char * to fix the warning.

Signed-off-by: Matthew Penney <matt@matthewpenney.net>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>