test: flaky replication/prune.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.6.0-114-g665543369d
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 (Debian 9)

Bug description:

1. https://gitlab.com/tarantool/tarantool/-/jobs/759350476#L5047

artifacts.zip

results file checksum: 2dd52fed642b0132ccef3853ad96a807

[066] --- replication/prune.result	Sat Sep 26 20:34:24 2020
[066] +++ replication/prune.reject	Sun Sep 27 06:39:27 2020
[066] @@ -163,7 +163,8 @@
[066]  ...
[066]  test_run:cmd('eval replica1 "box.info.replication[1].upstream.message"')[1]:match("Can't subscribe non%-anonymous replica") ~= nil
[066]  ---
[066] -- true
[066] +- error: '[string "return test_run:cmd(''eval replica1 "box.info...."]:1: ''void *''
[066] +    has no member named ''match'''
[066]  ...
[066]  replica_set.delete(test_run, 2)
[066]  ---
[066] 

2. https://gitlab.com/tarantool/tarantool/-/jobs/760389507

artifacts.zip

results file checksum: 7efe2d7cb3a869a1ea5fb954f323d06d

[007] --- replication/prune.result	Mon Sep 28 08:33:00 2020
[007] +++ replication/prune.reject	Mon Sep 28 10:43:51 2020
[007] @@ -159,7 +159,7 @@
[007]  ...
[007]  test_run:cmd('eval replica1 "box.info.replication[1].upstream.status"')
[007]  ---
[007] -- ['loading']
[007] +- ['connect']
[007]  ...
[007]  test_run:cmd('eval replica1 "box.info.replication[1].upstream.message"')[1]:match("Can't subscribe non%-anonymous replica") ~= nil
[007]  ---
[007] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat