test: flaky box/on_shutdown.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.7.0-50-g993019d12
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 -Wno-gnu-alignof-expression -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 -Wno-invalid-offsetof -Wno-gnu-alignof-expression -Werror

OS version:
OSX 14

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/869233631#L4789

results file checksum: 4c18b44583b1a169f0fff5baed531efd

[017] --- box/on_shutdown.result	Thu Nov 19 20:31:06 2020
[017] +++ /tmp/tnt/rejects/box/on_shutdown.reject	Tue Nov 24 15:30:02 2020
[017] @@ -1,169 +1,3 @@
[017] -env = require('test_run')
[017]  ---
[017] +- error: Can't drop the primary key in a system space, space '_schema'
[017]  ...
[017] -log = require('log')
[017] ----
[017] -...

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat