test: flaky replication/qsync_advanced.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.6.0-112-g59dca1898c
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:
Linux (Debug 9)

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/758318534#L4572

artifacts.zip

results file checksum: 51ee48072d103509eca347ecfc4ca26a

[044] --- replication/qsync_advanced.result	Fri Sep 25 18:07:04 2020
[044] +++ replication/qsync_advanced.reject	Fri Sep 25 19:01:31 2020
[044] @@ -604,11 +604,11 @@
[044]  -- Testcase body.
[044]  box.space.sync:insert{1}
[044]   | ---
[044] - | - [1]
[044] - | ...
[044] -box.space.sync:select{} -- 1
[044] - | ---
[044] - | - - [1]
[044] + | - error: Quorum collection for a synchronous transaction is timed out
[044] + | ...
[044] +box.space.sync:select{} -- 1
[044] + | ---
[044] + | - []
[044]   | ...
[044]  test_run:switch('replica')
[044]   | ---
[044] @@ -636,7 +636,7 @@
[044]   | ...
[044]  box.space.sync:select{} -- 1
[044]   | ---
[044] - | - - [1]
[044] + | - []
[044]   | ...
[044]  -- Testcase cleanup.
[044]  test_run:switch('default')
[044] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat