test: flaky replication/qsync_basic test

[avtikhon]

Tarantool version:
Tarantool 2.6.0-115-g8f3470399
Target: Linux-x86_64-RelWithDebInfo
Build options: cmake . -DCMAKE_INSTALL_PREFIX=/usr/local -DENABLE_BACKTRACE=ON
Compiler: /usr/bin/clang /usr/bin/clang++
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:
Linux (Debian 9)

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/760042837#L4419

artifacts.zip

results file checksum: aa0f68d40ef64fa9690d20e652d7f47c

[058] --- replication/qsync_basic.result	Fri Sep 25 16:53:45 2020
[058] +++ replication/qsync_basic.reject	Mon Sep 28 06:08:48 2020
[058] @@ -495,7 +495,7 @@
[058]   | ...
[058]  box.space.sync:select{7}
[058]   | ---
[058] - | - - [7]
[058] + | - []
[058]   | ...
[058]  
[058]  --
[058] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat