selftests: mptcp: add a check for 'add_addr_accepted'

selftests: mptcp: add a check for 'add_addr_accepted'

The previous patch fixed an issue with the 'add_addr_accepted' counter.
This was not spot by the test suite.

Check this counter and 'add_addr_signal' in MPTCP Join 'delete re-add
signal' test. This should help spotting similar regressions later on.
These counters are crucial for ensuring the MPTCP path manager correctly
handles the subflow creation via 'ADD_ADDR'.

Signed-off-by: Gang Yan <yangang@kylinos.cn>
Reviewed-by: Geliang Tang <geliang@kernel.org>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251118-net-mptcp-misc-fixes-6-18-rc6-v1-11-806d3781c95f@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>