test: fix net.box occasional failure. Again

The test regarding logging corrupted rows failed occasionally with
```
[016]  test_run:grep_log('default', 'Got a corrupted row.*')
[016]  ---
[016] -- 'Got a corrupted row:'
[016] +- null
[016]  ...
```
The logs then had
```
[010] 2019-07-06 19:36:16.857 [13046] iproto sio.c:261 !> SystemError writev(1),
called on fd 23, aka unix/:(socket), peer of unix/:(socket): Broken pipe
```
instead of the expected message.

This happened, because we closed a socket before tarantool could write a
greeting to the client, the connection was then closed, and execution
never got to processing the malformed request and thus printing the
desired message to the log.

To fix this, actually read the greeting prior to writing new data and
closing the socket.

Follow-up #4273