test: box/net.box flaky fails on finding the 'timed out' error

[avtikhon]

Tarantool version:
master

OS version:
Debian/Ubuntu

Bug description:

[004] Test failed! Result content mismatch:
[004] --- box/net.box.1533.result	Thu Jul 11 15:10:28 2019
[004] +++ box/net.box.1533.reject	Thu Jul 11 15:11:34 2019
[004] @@ -53,7 +53,7 @@
[004]   | ...
[004]  nb.error:find('timed out') ~= nil;
[004]   | ---
[004] - | - true
[004] + | - false
[004]   | ...
[004]  nb:close();
[004]   | ---

Steps to reproduce:
Run near to 10 loops or more to get the issue:

l=0 ; while ./test-run.py -j40 `for r in {1..100} ; do echo box/net.box.1533.test.lua ; done` 2>/dev/null ; do l=$(($l+1)) ; echo ======== $l ============= ; done

Reproducer:

net = require('net.box')
fiber = require 'fiber'
test_run = require('test_run').new()

-- Tarantool < 1.7.1 compatibility (gh-1533)
c = net.new(box.cfg.listen)
c:ping()
c:close()

-- Test for connect_timeout > 0 in netbox connect
test_run:cmd("setopt delimiter ';'");
greeting =
"Tarantool 1.7.3 (Lua console)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n" ..
"type 'help' for interactive help~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n";
socket = require('socket');
srv = socket.tcp_server('localhost', 0, {
    handler = function(fd)
        local fiber = require('fiber')
        fiber.sleep(0.1)
        fd:write(greeting)
    end
});
port = srv:name().port
-- we must get timeout
nb = net.new('localhost:' .. port, {
    wait_connected = true, console = true,
    connect_timeout = 0.01
});
nb.error:find('timed out') ~= nil;
nb:close();
-- we must get peer closed
nb = net.new('localhost:' .. port, {
    wait_connected = true, console = true,
    connect_timeout = 0.2
});
nb.error ~= "Timeout exceeded";
nb:close();
test_run:cmd("setopt delimiter ''");
srv:close()

test_run:cmd("clear filter")

Optional (but very desirable):

• coredump
• backtrace
• netstat