test: flaky box/sequence.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/556301323

 [022] --- box/sequence.result	Sun May 17 15:37:55 2020
 [022] +++ box/sequence.reject	Mon May 18 03:08:12 2020
 [022] @@ -773,8 +773,8 @@
 [022]  ...
 [022]  box.space._index:update({s.id, pk.id}, {{'=', 6, {{0, 'string'}}}}) -- error
 [022]  ---
 [022] -- error: 'Can''t create or modify index ''pk'' in space ''test'': sequence cannot
 [022] -    be used with a non-integer key'
 [022] +- error: 'Can''t create or modify index ''pk'' in space ''test'': sequence field must
 [022] +    be a part of the index'
 [022]  ...
 [022]  box.space._index:delete{s.id, pk.id} -- error
 [022]  ---

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat