testsuite: make a sed command more portable

avoid using some GNU sed extensions, which are not available
eg. on NetBSD's /usr/bin/sed.

while there are other places these extenstions are used in the testsuite,
this instance is new and seems the only one currently being executed
for NetBSD.

Signed-off-by: YAMAMOTO Takashi <yamt@mwd.biglobe.ne.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>