test: flaky engine/tuple.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.7.0-9-ge75db7a499
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: -static-libstdc++ -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: -static-libstdc++ -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/821298876#L5401

artifacts.zip

results file checksum: 8fdb38e170c7c1e0c2353a0e76547081

[006] --- engine/tuple.result	Fri Oct 23 07:52:00 2020
[006] +++ var/rejects/engine/tuple.reject	Fri Oct 30 12:34:30 2020
[006] @@ -70,6 +70,7 @@
[006]  ...
[006]  _ = s:replace({1, string.rep('x', 1 * 1024 * 1024 - engine_tuple_size - 8)})
[006]  ---
[006] +- error: Timed out waiting for Vinyl memory quota
[006]  ...
[006]  -- tarantool/tarantool#2698 Tarantool crashed on 4M tuple
[006]  max_item_size = 0
[006] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat