test: flaky box/transaction.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.5.0-32-g7f20272ea
Target: Darwin-x86_64-RelWithDebInfo
Build options: cmake . -DCMAKE_INSTALL_PREFIX=/usr/local -DENABLE_BACKTRACE=ON
Compiler: /Library/Developer/CommandLineTools/usr/bin/cc /Library/Developer/CommandLineTools/usr/bin/c++
C_FLAGS: -Wno-unknown-pragmas -fexceptions -funwind-tables -fno-omit-frame-pointer -fno-stack-protector -fno-common -msse2 -std=c11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Werror
CXX_FLAGS: -Wno-unknown-pragmas -fexceptions -funwind-tables -fno-omit-frame-pointer -fno-stack-protector -fno-common -msse2 -std=c++11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Werror

OS version:
OSX

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/555266320

[012] box/transaction.test.lua                                        [ fail ]
[012] 
[012] Test failed! Result content mismatch:
[012] --- box/transaction.result	Sat May 16 01:05:37 2020
[012] +++ box/transaction.reject	Sat May 16 04:10:16 2020
[012] @@ -839,6 +839,7 @@
[012]      fiber.sleep(0)
[012]  end;
[012]  ---
[012] +- error: 'Supplied key type of part 1 does not match index part type: expected string'
[012]  ...
[012]  test_run:cmd("setopt delimiter ''");
[012]  ---

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat