selftests: mptcp: export TEST_COUNTER variable

Variable TEST_COUNT are used in mptcp_connect.sh and mptcp_join.sh as
test counters, which are initialized to 0, while variable test_cnt are used
in diag.sh and simult_flows.sh, which are initialized to 1. To maintain
consistency, this patch renames them all as MPTCP_LIB_TEST_COUNTER,
initializes it to 1, and exports it into mptcp_lib.sh.

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-v1-5-4f42c347b653@kernel.org


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