test: fix reading STDIN command on openSUSE

Inspired by tarantoolgh-5064, that breaks the previous version of the test on
openSUSE. When using `io.popen:write()` on tarantool with `-i` flag, it
failed to run the command on openSUSE. This happened because before
tarantoolgh-5064 patch it used to employ `luaL_loadfile()` that interprets EOF
as the end of the command, while when it is loaded as a string openSUSE
expects it to end with '\n'.

Needed for tarantool#5064
NO_DOC=test fix
NO_TEST=test fix
NO_CHANGELOG=test fix

test/box/gh-4703-on_shutdown-bug.result

@@ -22,7 +22,7 @@ test_run:cmd("setopt delimiter ';'");
  | ...
 on_shutdown_cmd = "box.ctl.on_shutdown(function() local fio = require('fio') "..
                    "fio.open('"..file_name.."', "..
-                   "{'O_CREAT', 'O_TRUNC', 'O_WRONLY'}, 777):close() end)";
+                   "{'O_CREAT', 'O_TRUNC', 'O_WRONLY'}, 777):close() end)\n";
  | ---
  | ...
 test_run:cmd("setopt delimiter ''");

test/box/gh-4703-on_shutdown-bug.test.lua

@@ -8,9 +8,10 @@ test_run = env.new()
 --
 file_name = "on_shutdown_triggered.txt"
 test_run:cmd("setopt delimiter ';'");
+-- Extra \n is required on openSUSE.
 on_shutdown_cmd = "box.ctl.on_shutdown(function() local fio = require('fio') "..
                    "fio.open('"..file_name.."', "..
-                   "{'O_CREAT', 'O_TRUNC', 'O_WRONLY'}, 777):close() end)";
+                   "{'O_CREAT', 'O_TRUNC', 'O_WRONLY'}, 777):close() end)\n";
 test_run:cmd("setopt delimiter ''");
 server = io.popen('tarantool -i', 'w')
 server:write(on_shutdown_cmd)