selftests: mptcp: connect: don't stop if error

No more tests were executed after a failure but it is still interesting
to get results for all the tests to better understand what's still OK
and what's not after a modification.

Now we only exit earlier if the basic tests are failing: no ping going
through namespaces or unable to transfer data on the loopback interface.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>