test: flaky box/net.box_incompatible_index-gh-1729.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.6.0-114-g0670099a7
Target: Darwin-x86_64-RelWithDebInfo
Build options: cmake . -DCMAKE_INSTALL_PREFIX=/Users/tntmac04.tarantool.i/builds/PVWCJs8U/0/tarantool/tarantool/static-build/tarantool-prefix -DENABLE_BACKTRACE=TRUE
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/759595313#L5168

artifacts.zip

results file checksum: 1390e6d8adfa1b29a05df031f91884c6

[025] --- box/net.box_incompatible_index-gh-1729.result	Wed Aug 26 06:06:15 2020
[025] +++ box/net.box_incompatible_index-gh-1729.reject	Sun Sep 27 19:15:35 2020
[025] @@ -16,6 +16,8 @@
[025]  ...
[025]  _ = box.space.test:create_index('covering', {type = 'TREE', parts = {1,'unsigned',3,'string',2,'unsigned'}})
[025]  ---
[025] +- error: 'Can''t create or modify index ''covering'' in space ''test'': same key part
[025] +    is indexed twice'
[025]  ...
[025]  _ = box.space.test:insert{1, 2, "string"}
[025]  ---
[025] @@ -31,19 +33,12 @@
[025]  ---
[025]  - - type: unsigned
[025]      is_nullable: false
[025] -    fieldno: 1
[025] +    fieldno: 2
[025]  ...
[025]  c.space.test.index.covering.parts
[025]  ---
[025] -- - type: unsigned
[025] -    is_nullable: false
[025] -    fieldno: 1
[025] -  - type: string
[025] -    is_nullable: false
[025] -    fieldno: 3
[025] -  - type: unsigned
[025] -    is_nullable: false
[025] -    fieldno: 2
[025] +- error: '[string "return c.space.test.index.covering.parts "]:1: attempt to index
[025] +    field ''covering'' (a nil value)'
[025]  ...
[025]  box.space.test:drop()
[025]  ---
[025] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat