Draft: New r202/speedup_untimed_socket_read subtest verifying speed up untimed SOCKET READ when socket device has no DELIMITER

Release note YDB#1100 (closed) says:

Untimed READ commands on SOCKET devices with no DELIMITER return as soon as some data is available. This has been seen to improve socket READ performance by avoiding time spent unnecessarily sleeping/waiting for more data to be available. Previously, they used to indefinitely wait for a huge amount of data (32Kib) to be read before returning from the READ command. See, for example, YDBXider!50 (merged) (merged) for a 30% improvement seen in YDBXider server.