selftests: Use separate stdout and stderr buffers in nettest

When a single instance of nettest is doing both client and
server modes, stdout and stderr messages can get interlaced
and become unreadable. Allocate a new set of buffers for the
child process handling server mode.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>