test: flaky vinyl/upsert.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.6.0-142-g72fc325fc
Target: Linux-x86_64-RelWithDebInfo
Build options: cmake . -DCMAKE_INSTALL_PREFIX=/usr -DENABLE_BACKTRACE=ON
Compiler: /usr/bin/cc /usr/bin/c++
C_FLAGS:-O2 -g -m64 -fmessage-length=0 -D_FORTIFY_SOURCE=2 -fstack-protector -funwind-tables -fasynchronous-unwind-tables -DNDEBUG -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
CXX_FLAGS:-O2 -g -m64 -fmessage-length=0 -D_FORTIFY_SOURCE=2 -fstack-protector -funwind-tables -fasynchronous-unwind-tables -DNDEBUG -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

OS version:
openSUSE 15.2

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/771784237#L2866

artifacts.zip

results file checksum: c78c48339cd492202dc4d882435e94dd

[058] --- vinyl/upsert.result	Sun Oct  4 17:12:04 2020
[058] +++ vinyl/upsert.reject	Sun Oct  4 17:15:13 2020
[058] @@ -830,7 +830,7 @@
[058]  ...
[058]  ch:get() -- should see the UPSERT and return [10, 20]
[058]  ---
[058] -- - [10, 20]
[058] +- - [10, 10]
[058]  ...
[058]  s:drop()
[058]  ---
[058] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat