selftests: validate listener sockets with multiple accepts

[matttbe]

We are currently one validating the creation of one MPTCP connection at a time. We don't look at the behaviour of listener sockets having multiple accepts.

One particular case that is interesting to validate is when some accepted sockets have done a fallback to TCP and some no.

[matttbe]

Suggested by @pabeni (don't hesitate to edit the description :) )