glibc/check-test-wrapper: don't emit warnings from ssh

Dont fill up the test log with ssh warning about having added the host
to list of known hosts.

Also helps fix a test case failure where stderr log was being compared
to a known value.

Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 63b31ff7)
Signed-off-by: Steve Sakoman <steve@sakoman.com>