selftests: mptcp: add speed env var

Use a new env var speed instead of using 'speed' argument of run_tests()
and do_transfer(). It can be set with 'fast', 'slow', or the actual speed
number:

  run_tests $ns1 $ns2 10.0.1.1 slow

  ->

  speed=slow \
          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-v1-4-f1c8b62fbf95@tessares.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>