test/zdtm.sh: move -h parsing to inside case

It's cleaner to have all options parsed in the same way (and in the same
manner).

Factor out usage() and call it when -h is found.

Also, print usage text to stdout not stderr.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>