selftests: mptcp: declare event macros in mptcp_lib

MPTCP event macros (SUB_ESTABLISHED, LISTENER_CREATED, LISTENER_CLOSED),
and the protocol family macros (AF_INET, AF_INET6) are defined in both
mptcp_join.sh and userspace_pm.sh. In order not to duplicate code, this
patch declares them all in mptcp_lib.sh with MPTCP_LIB_ prefixs.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240308-upstream-net-next-20240308-selftests-mptcp-unification-v1-14-4f42c347b653@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>