selftests: mptcp: join: fix 'implicit EP' test

selftests: mptcp: join: fix 'implicit EP' test

mptcp_join 'implicit EP' test currently fails when using ip mptcp:

  $ ./mptcp_join.sh -iI
  <snip>
  001 implicit EP    creation[fail] expected '10.0.2.2 10.0.2.2 id 1 implicit' found '10.0.2.2 id 1 rawflags 10 '
  Error: too many addresses or duplicate one: -22.
                     ID change is prevented[fail] expected '10.0.2.2 10.0.2.2 id 1 implicit' found '10.0.2.2 id 1 rawflags 10 '
                     modif is allowed[fail] expected '10.0.2.2 10.0.2.2 id 1 signal' found '10.0.2.2 id 1 signal '

This happens because of two reasons:
- iproute v6.3.0 does not support the implicit flag, fixed with
  iproute2-next commit 3a2535a41854 ("mptcp: add support for implicit
  flag")
- pm_nl_check_endpoint wrongly expects the ip address to be repeated two
  times in iproute output, and does not account for a final whitespace
  in it.

This fixes the issue trimming the whitespace in the output string and
removing the double address in the expected string.

Fixes: 69c6ce7b6eca ("selftests: mptcp: add implicit endpoint test case")
Cc: stable@vger.kernel.org
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20230803-upstream-net-20230803-misc-fixes-6-5-v1-2-6671b1ab11cc@tessares.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>