selftests: mptcp: add missing join check

This function also writes the name of the test with its ID, making clear
a new test has been executed.

Without that, the ADD_ADDR results from this test was appended at the
end of the previous test causing confusions. Especially when the second
test was failing, we had:

  17 signal invalid addresses     syn[ ok ] - synack[ ok ] - ack[ ok ]
                                  add[ ok ] - echo  [ ok ]
                                  add[fail] got 2 ADD_ADDR[s] expected 3

In fact, this 17th test was OK but not the 18th one.

Now we have:

  17 signal invalid addresses     syn[ ok ] - synack[ ok ] - ack[ ok ]
                                  add[ ok ] - echo  [ ok ]
  18 signal addresses race test   syn[fail] got 2 JOIN[s] syn expected 3
   - synack[fail] got 2 JOIN[s] synack expected
   - ack[fail] got 2 JOIN[s] ack expected 3
                                  add[fail] got 2 ADD_ADDR[s] expected 3

Fixes: 33c563ad28 ("selftests: mptcp: add_addr and echo race test")
Reported-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/mptcp/mptcp_join.sh

@@ -1159,6 +1159,7 @@ signal_address_tests()
 	ip netns exec $ns2 ./pm_nl_ctl add 10.0.3.2 flags signal
 	ip netns exec $ns2 ./pm_nl_ctl add 10.0.4.2 flags signal
 	run_tests $ns1 $ns2 10.0.1.1
+	chk_join_nr "signal addresses race test" 3 3 3
 
 	# the server will not signal the address terminating
 	# the MPC subflow