test: flaky box/tree_pk_multipart.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.7.0-26-g8dcbf7920
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 13

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/835837955#L4794

artifacts.zip

results file checksum: a30bba1908e52bc9fc31f6c33565cdc1

[034] --- box/tree_pk_multipart.result	Wed Nov  4 15:10:50 2020
[034] +++ /tmp/tnt/rejects/box/tree_pk_multipart.reject	Sat Nov  7 07:36:35 2020
[034] @@ -687,7 +687,7 @@
[034]  ...
[034]  good
[034]  ---
[034] -- true
[034] +- false
[034]  ...
[034]  s1:drop()
[034]  ---
[034] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat