openssh: improve banner ptest failure logging

openssh: improve banner ptest failure logging

Log the input and output banner files. Output seems to
contain more lines than input which fails the test but
it's not clear what is in there from the ssh command
stderr. So print them out to dig deeper into the root
cause.

Upstream rejected previous logging patch so they will likely
do the same for this:
https://github.com/openssh/openssh-portable/pull/437

Reference: https://bugzilla.yoctoproject.org/show_bug.cgi?id=15178

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>