selftests: mptcp: join: tolerate more ADD_ADDR

selftests: mptcp: join: tolerate more ADD_ADDR

ADD_ADDR can be retransmitted, and with, the parent commit, these
retransmissions can be sent quicker: from 2 minutes to less than one
second.

To avoid false positives where retransmitted ADD_ADDR causes higher
counters than expected, it is required to be more tolerant. Errors are
now only reported when fewer ADD_ADDRs have been sent/received, except
if no ADD_ADDR are expected.

Before the parent commit, the tolerance was present for each tests where
the ADD_ADDR could be retransmitted in a reasonable time (1 sec). Now
that all tests can have retransmitted ADD_ADDR, it is normal to apply
the same tolerance for all tests.

An alternative could be to disable the ADD_ADDR retransmissions by
default, but that's changing the default kernel behaviour. Plus,
ADD_ADDR retransmissions can be required for some tests. To avoid adding
exceptions to many tests, it seems better to increase the tolerance.

Later, we could add a new MIB counter to identify the ADD_ADDR
retransmissions, and remove the tolerance when this counter is
available.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250907-net-next-mptcp-add_addr-retrans-adapt-v1-2-824cc805772b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>