selftests: mptcp: drop test_linkfail parameter

run_tests() accepts too many optional parameters. Before this modification,
it was required to set all of then when only the last one had to be
changed. That's not clear to see all these 0 and it makes the maintenance
harder:

      run_tests $ns1 $ns2 10.0.1.1 1 2 3 slow

Instead, the parameter can be set as an env var with a limited scope:

      foo=1 bar=2 next=3 \
            run_tests $ns1 $ns2 10.0.1.1 slow

This patch switches to key/value "test_linkfail=*" instead of positional
parameter test_linkfail of do_transfer() and run_tests().

Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20230623-send-net-next-20230623-v1-4-a883213c8ba9@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>