test: flaky engine/conflict.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.7.0-42-g57e05c1c3d
Target: Linux-x86_64-Debug
Build options: cmake . -DCMAKE_INSTALL_PREFIX=/usr/local -DENABLE_BACKTRACE=ON
Compiler: /usr/bin/cc /usr/bin/c++
C_FLAGS: -fexceptions -funwind-tables -fno-omit-frame-pointer -fno-stack-protector -fno-common -fopenmp -msse2 -fprofile-arcs -ftest-coverage -std=c11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Wno-format-truncation -Wno-gnu-alignof-expression -fno-gnu89-inline -Wno-cast-function-type -Werror
CXX_FLAGS: -fexceptions -funwind-tables -fno-omit-frame-pointer -fno-stack-protector -fno-common -fopenmp -msse2 -fprofile-arcs -ftest-coverage -std=c++11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Wno-format-truncation -Wno-invalid-offsetof -Wno-gnu-alignof-expression -Wno-cast-function-type -Werror

OS version:
Debian 9

Bug description:

1. https://gitlab.com/tarantool/tarantool/-/jobs/847373920#L4589

artifacts.zip

results file checksum: 187434595fcf4e4d22f2ecee707f5e50

[036] --- engine/conflict.result	Fri Jun  5 06:49:36 2020
[036] +++ var/rejects/engine/conflict.reject	Fri Nov 13 08:42:08 2020
[036] @@ -3,5 +3,5 @@
[036]  ...
[036]  test_conflict()
[036]  ---
[036] -- 2
[036] +- 1
[036]  ...
[036] 

2. https://gitlab.com/tarantool/tarantool/-/jobs/847027505#L4374

results file checksum: 20842c99301b23e71a6365550333da87

[065] --- engine/conflict.result	Fri Nov 13 03:07:03 2020
[065] +++ /rw_bins/test/var/rejects/engine/conflict.reject	Fri Nov 13 03:09:29 2020
[065] @@ -3,5 +3,5 @@
[065]  ...
[065]  test_conflict()
[065]  ---
[065] -- 2
[065] +- 0
[065]  ...
[065] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat