Sanity tests related to number of packets, packet size & packet priority

Summary:
related to T12015
- add tests related to sending/receiving really big packets
- add tests related to receiving multiple packets at once
- send_big_publish fails in the testing environment, but it works fine in real life

Reviewers: ivica

Reviewed By: ivica

Subscribers: miljen, iljazovic

Differential Revision: https://repo.mireo.local/D27652

test/include/test_common/packet_util.hpp

@@ -343,10 +343,8 @@ inline std::string to_readable_packet(std::string packet) {
 		case control_code_e::unsuback:
 			return detail::to_string<control_code_e::unsuback>(*varlen, begin);
 		default:
-			assert(false);
+			return "";
 	}
-
-	return {};
 }
 
 template <typename ConstBufferSequence>