tests/unix_seqpacket: provide function that returns bound & listening sock

[glebius]

Summary

Use the function in mk_pair_of_sockets(), and in two tests -
'listen_bound' and 'connect'. While here make mk_pair_of_sockets() return
pointer to sockaddr_un instead of path. This also fixes bug of returning
pointer to stack memory of returning function. However, the only caller
that cares about this return is temporarily ifdefed out. No functional
changes intended.

[glebius]

Couple more places to use the new function.