selftests: mptcp: join: cover ADD_ADDR tx drop and list progress

selftests: mptcp: join: cover ADD_ADDR tx drop and list progress

Extend add_addr_ports_tests with IPv6 signaling cases that exercise
ADD_ADDR tx-space shortage when tcp_timestamps are enabled.

Add one case to verify PM still progresses to later signal endpoints
after the first one is dropped.

This covers both failure accounting and the non-blocking behavior of
the announce list after a tx-space drop on pure ACK.

Signed-off-by: Li Xiasong <lixiasong1@huawei.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260515-net-mptcp-misc-fixes-7-1-rc4-v2-3-701e96419f2f@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>