test: flaky swim/swim.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.7.0-71-g1bffedb748
Target: Linux-x86_64-RelWithDebInfo
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 -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 -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/862298315#L5323

artifacts.zip

results file checksum: 867bb2d251a387c53875442f71410e7f

[109] --- swim/swim.result	Mon Nov  9 05:47:59 2020
[109] +++ var/rejects/swim/swim.reject	Fri Nov 20 04:41:12 2020
[109] @@ -238,9 +238,13 @@
[109]  ...
[109]  while s2:size() ~= 2 or s1:size() ~= 2 do fiber.sleep(0.01) s1:broadcast(listen_port) end
[109]  ---
[109] +- error: '[string "while s2:size() ~= 2 or s1:size() ~= 2 do fib..."]:1: attempt to
[109] +    index global ''s2'' (a nil value)'
[109]  ...
[109]  s2:delete()
[109]  ---
[109] +- error: '[string "return s2:delete() "]:1: attempt to index global ''s2'' (a nil
[109] +    value)'
[109]  ...
[109]  s1.remove_member()
[109]  ---
[109] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat