selftests: mptcp: join: reset failing links

Best to always reset this env var before each test to avoid surprising
behaviour depending on the order tests are running.

Also clearly set it for the last failing links test is also needed when
only this test is executed.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>