tests: Avoid nonportable use of bash-specific {a..b..c} construct.

Reported-by: Alex Wang <alexw@nicira.com>
Reported-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>