selftests: mptcp: join: option to execute specific tests

Often, it is needed to run one specific test.

There are options to run subgroups of tests but when only one fails, no
need to run all the subgroup. So far, the solution was to edit the
script to comment the tests that are not needed but that's not ideal.

Now, it is possible to run one specific test by giving the ID of the
tests that are going to be validated, e.g.

  ./mptcp_join.sh 36 37

This is cleaner and saves time.

Technically, the reset* functions now return 0 if the test can be
executed. This naturally creates sections per test in the code which is
also helpful to understand what a test is exactly doing.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
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>