selftests: mptcp: add fastclose env var

Use a new env var fastclose instead of passing fastclose to addr_nr_ns2.
It can be set with 'server' or 'client':

  addr_nr_ns2=fastclose_client \
          run_tests $ns1 $ns2 10.0.1.1

  ->

  fastclose=client \
          run_tests $ns1 $ns2 10.0.1.1.

With this change, the fullmesh flag setting code can be moved into
pm_nl_set_endpoint() from do_transfer().

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-v1-2-f1c8b62fbf95@tessares.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>