net: lib: coap_client: check poll() condition before retrying CoAP msg

Refactor the CoAP retry handling into the handle_poll() function,
so that we only try to send retries if the socket reports POLLOUT.

Also move the receiving into same loop, so when poll() reports POLLIN
we recv() the message and handle it before proceeding to other sockets.

Also fix tests to handle POLLOUT flag and add support for testing
multiple clients.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>