Bluetooth: l2cap: change `connected` cb call condition for ECRED

This was recently refactored (in #58440). But it introduced a bug in which
some channels were connected but not the whole list asked for by the peer.

In that case, `result` will not be `SUCCESS` but we still want to call the
`connected` callback as the peer will consider those channels to be
connected when we send the response.

The symptom is that EATT channels are being instantiated, but not
considered connected (ie. usable by the stack).

Also introduce a test that has asymmetric channel resources (5 on central
vs 2 on peripheral) to reproduce the bug.

Fixes #60212

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>