selftests: mptcp: add print_title in mptcp_lib

This patch adds a new variable MPTCP_LIB_TEST_FORMAT as the test title
printing format. Also add a helper mptcp_lib_print_title() to use this
format to print the test title with test counters. They are used in
mptcp_join.sh first.

Each MPTCP selftest is having subtests, and it helps to give them a
number to quickly identify them. This can be managed by mptcp_lib.sh,
reusing what has been done here. The following commit will use these
new helpers in the other tests.

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-v1-6-4f42c347b653@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>