tests: coap_client: Add test case for resending request

Add case 'test_resend_request' to test that coap_client resends
the message if no response received.
Add response code check to case 'test_no_response'.
Initialize messages_needing_response[] in test setup.
Lower CONFIG_COAP_INIT_ACK_TIMEOUT_MS and decrease delays.

Signed-off-by: Juha Ylinen <juha.ylinen@nordicsemi.no>