selftests: mptcp: interpret \n as a new line

In case of errors, this message was printed:

  (...)
  balanced bwidth with unbalanced delay       5233 max 5005  [ fail ]
  client exit code 0, server 0
  \nnetns ns3-0-EwnkPH socket stat for 10003:
  (...)

Obviously, the idea was to add a new line before the socket stat and not
print "\nnetns".

The commit 8b974778 ("selftests: mptcp: interpret \n as a new line")
is very similar to this one. But the modification in simult_flows.sh was
missed because this commit above was done in parallel to one here below.

Fixes: 1a418cb8 ("mptcp: simult flow self-tests")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>