test: flaky box/call.test.lua test

[avtikhon]

Tarantool version:
| Tarantool 2.7.0-74-g6ad720d6d
| Target: Darwin-x86_64-RelWithDebInfo
| Build options: cmake . -DCMAKE_INSTALL_PREFIX=/usr/local -DENABLE_BACKTRACE=ON
| Compiler: /Library/Developer/CommandLineTools/usr/bin/cc /Library/Developer/CommandLineTools/usr/bin/c++
| 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:
OSX

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/875825810#L4625

artifacts.zip

results file checksum: 10288ad585ec99f0e8d121d6a933ab5e

[010] box/call.test.lua                                               [ fail ]
[010] 
[010] Test failed! Result content mismatch:
[010] --- box/call.result	Fri Nov  6 11:35:25 2020
[010] +++ /tmp/tnt/rejects/box/call.reject	Fri Nov 27 03:06:08 2020
[010] @@ -591,8 +591,7 @@
[010]  ...
[010]  conn:call_16("return_serialize7")
[010]  ---
[010] -- - [1, null, null, null, null, null, null, null, null, null, null, null, null, null,
[010] -    null, null, null, null, null, 1]
[010] +- - [1, null]
[010]  ...
[010]  function return_serialize8() return { tomap({ 'a', 'b', 'c'})} end
[010]  ---

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat