selftests: mptcp: join: alt. to exec specific tests

Running a specific test by giving the ID is often what we want: the CI
reports an issue with the Nth test, it is reproducible with:

  ./mptcp_join.sh N

But this might not work when there is a need to find which commit has
introduced a regression making a test unstable: failing from time to
time. Indeed, a specific test is not attached to one ID: the ID is in
fact a counter. It means the same test can have a different ID if other
tests have been added/removed before this unstable one.

Remembering the current test can also help listing failed tests at the
end.
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>