selftests: mptcp: more robust signal race test

The in kernel MPTCP PM implementation can process a single
incoming add address option at any given time. In the
mentioned test the server can surpass such limit. Let the
setup cope with that allowing a faster add_addr retransmission.

Fixes: a88c9e49 ("mptcp: do not block subflows creation on errors")
Fixes: f7efc777

 ("mptcp: drop argument port from mptcp_pm_announce_addr")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/254
Reported-and-tested-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>