selftests: mptcp: join: validate backup in MPJ

A peer can notify the other one that a subflow has to be treated as
"backup" by two different ways: either by sending a dedicated MP_PRIO
notification, or by setting the backup flag in the MP_JOIN handshake.

The selftests were previously monitoring the former, but not the latter.
This is what is now done here by looking at these new MIB counters when
validating the 'backup' cases:

  MPTcpExtMPJoinSynBackupRx
  MPTcpExtMPJoinSynAckBackupRx

The 'Fixes' tag here below is the same as the one from the previous
commit: this patch here is not fixing anything wrong in the selftests,
but it will help to validate a new fix for an issue introduced by this
commit ID.

Fixes: 4596a2c1

 ("mptcp: allow creating non-backup subflows")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>