selftests: mptcp: join: check backup support in signal endp

Before the previous commit, 'signal' endpoints with the 'backup' flag
were ignored when sending the MP_JOIN.

The MPTCP Join selftest has then been modified to validate this case:
the "single address, backup" test, is now validating the MP_JOIN with a
backup flag as it is what we expect it to do with such name. The
previous version has been kept, but renamed to "single address, switch
to backup" to avoid confusions.

The "single address with port, backup" test is also now validating the
MPJ with a backup flag, which makes more sense than checking the switch
to backup with an MP_PRIO.

The "mpc backup both sides" test is now validating that the backup flag
is also set in MP_JOIN from and to the addresses used in the initial
subflow, using the special ID 0.

The 'Fixes' tag here below is the same as the one from the previous
commit: this patch here is not fixing anything wrong in the selftests,
but it validates the previous fix for an issue introduced by this commit
ID.

Fixes: 4596a2c1

 ("mptcp: allow creating non-backup subflows")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>