selftests: mptcp: add fullmesh env var

Use a new env var fullmesh instead of passing 'fullmesh_*' to addr_nr_ns2.
It can be set with the actual value of addr_nr_ns2 now:

  addr_nr_ns2=fullmesh_1 \
          run_tests $ns1 $ns2 10.0.1.1

  ->

  fullmesh=1 \
          run_tests $ns1 $ns2 10.0.1.1.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20230712-upstream-net-next-20230712-selftests-mptcp-use-local-env-v1-3-f1c8b62fbf95@tessares.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>