tools/l2cap-tester: add test for too many ECRED deferred sockets

Add test tries to trigger connection with 5+1 ECRED channels, and
require the last connect() shall fail with -EPROTO.  On some kernels
this causes invalid ECRED REQ be sent with 6 SCID, and stack overwrite.

L2CAP Ext-Flowctl Client - Defer limit