test: fix hardcoded port in box/net.box.test.lua

It allows to run the test many times in parallel to investigate flaky
test failures and decreases probability that the test fails, because
this port was already used by, say, some other test.

test/box/net.box.result

@@ -1880,7 +1880,7 @@ greeting =
 socket = require('socket');
 ---
 ...
-srv = socket.tcp_server('localhost', 3392, {
+srv = socket.tcp_server('localhost', 0, {
     handler = function(fd)
         local fiber = require('fiber')
         fiber.sleep(0.1)
@@ -1890,7 +1890,8 @@ srv = socket.tcp_server('localhost', 3392, {
 ---
 ...
 -- we must get timeout
-nb = net.new('localhost:3392', {
+port = srv:name().port
+nb = net.new('localhost:' .. port, {
     wait_connected = true, console = true,
     connect_timeout = 0.01
 });
@@ -1904,7 +1905,7 @@ nb:close();
 ---
 ...
 -- we must get peer closed
-nb = net.new('localhost:3392', {
+nb = net.new('localhost:' .. port, {
     wait_connected = true, console = true,
     connect_timeout = 0.2
 });

test/box/net.box.test.lua

@@ -754,22 +754,23 @@ greeting =
 "Tarantool 1.7.3 (Lua console)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n" ..
 "type 'help' for interactive help~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n";
 socket = require('socket');
-srv = socket.tcp_server('localhost', 3392, {
+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:3392', {
+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:3392', {
+nb = net.new('localhost:' .. port, {
     wait_connected = true, console = true,
     connect_timeout = 0.2
 });