test: flaky box/net.box_index_unique_flag_gh-4091.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.7.0-48-g27fbff62e
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 15

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/852519363#L4969

artifacts.zip

results file checksum: 1f53f6926961c55931128ae1f3f7a03c

[035] --- box/net.box_index_unique_flag_gh-4091.result	Thu Nov 12 16:13:40 2020
[035] +++ /tmp/tnt/rejects/box/net.box_index_unique_flag_gh-4091.reject	Wed Nov 18 02:53:54 2020
[035] @@ -25,4 +25,5 @@
[035]  ...
[035]  space:drop()
[035]  ---
[035] +- error: 'Can''t drop space ''test'': the space has grants'
[035]  ...
[035] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat