test: flaky replication/gc.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.7.0-6-gb8d95dfc78
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:
https://gitlab.com/tarantool/tarantool/-/jobs/821274118#L4823

artifacts.zip

results file checksum: eba8a4dd4110f26e5189c24f117a1679

[070] --- replication/gc.result	Fri Jun  5 06:49:36 2020
[070] +++ replication/gc.reject	Fri Oct 30 12:13:46 2020
[070] @@ -404,6 +404,7 @@
[070]  ...
[070]  _ = s:auto_increment{}
[070]  ---
[070] +- error: Can't modify data because this instance is in read-only mode.
[070]  ...
[070]  box.snapshot()
[070]  ---
[070] @@ -411,6 +412,7 @@
[070]  ...
[070]  _ = s:auto_increment{}
[070]  ---
[070] +- error: Can't modify data because this instance is in read-only mode.
[070]  ...
[070]  box.snapshot()
[070]  ---
[070] @@ -418,6 +420,7 @@
[070]  ...
[070]  _ = s:auto_increment{}
[070]  ---
[070] +- error: Can't modify data because this instance is in read-only mode.
[070]  ...
[070]  box.snapshot()
[070]  ---
[070] @@ -425,12 +428,14 @@
[070]  ...
[070]  wait_xlog(3) or fio.listdir('./master')
[070]  ---
[070] -- true
[070] +- - 00000000000000000517.snap
[070] +  - '514'
[070]  ...
[070]  -- Delete the replica from the cluster table and check that
[070]  -- all xlog files are removed.
[070]  test_run:cleanup_cluster()
[070]  ---
[070] +- error: Can't modify data because this instance is in read-only mode.
[070]  ...
[070]  box.snapshot()
[070]  ---
[070] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat