The test case has two problems that appear from time to time and lead to
flaky fails. Those fails are look as shown below in a test-run output.

 | Test failed! Result content mismatch:
 | --- box/net.box.result	Mon Jun 24 17:23:49 2019
 | +++ box/net.box.reject	Mon Jun 24 17:51:52 2019
 | @@ -1404,7 +1404,7 @@
 |  ...
 |  test_run:grep_log('default', 'ER_INVALID_MSGPACK.*')
 | ---
 | -- 'ER_INVALID_MSGPACK: Invalid MsgPack - packet body'
 | +- 'ER_INVALID_MSGPACK: Invalid MsgPack - packet length'
 | ...
 | -- gh-983 selecting a lot of data crashes the server or hangs the
 | -- connection

'ER_INVALID_MSGPACK.*' regexp should match 'ER_INVALID_MSGPACK: Invalid
MsgPack - packet body' log message, but if it is not in a log file at a
time of grep_log() call (just don't flushed to the file yet) a message
produced by another test case can be matched ('ER_INVALID_MSGPACK:
Invalid MsgPack - packet length'). The fix here is to match the entire
message and check for the message periodically during several seconds
(use wait_log() instead of grep_log()).

Another problem is the race between writing a response to an iproto
socket on a server side and closing the socket on a client end. If
tarantool is unable to write a response, it does not produce the warning
re invalid msgpack, but shows 'broken pipe' message instead. We need
first grep for the message in logs and only then close the socket on a
client. The similar problem (with another test case) is described in
[1].

[1]: https://github.com/tarantool/tarantool/issues/4273#issuecomment-508939695

Closes: #4311