test: flaky vinyl/errinj_ddl.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.5.0-28-g11a46331d1
Target: Linux-x86_64-Debug
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 -fprofile-arcs -ftest-coverage -std=c11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Wno-format-truncation -fno-gnu89-inline -Wno-cast-function-type -Werror
CXX_FLAGS: -fexceptions -funwind-tables -fno-omit-frame-pointer -fno-stack-protector -fno-common -fopenmp -msse2 -fprofile-arcs -ftest-coverage -std=c++11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Wno-format-truncation -Wno-invalid-offsetof -Wno-cast-function-type -Werror

OS version:
Linux

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/555511190

 [020] --- vinyl/errinj_ddl.result	Fri May 15 19:32:16 2020
 [020] +++ vinyl/errinj_ddl.reject	Sat May 16 13:16:56 2020
 [020] @@ -102,7 +102,7 @@
 [020]  ...
 [020]  s.index.sk:stat().memory.rows
 [020]  ---
 [020] -- 27
 [020] +- 23
 [020]  ...
 [020]  test_run:cmd('restart server default')
 [020]  fiber = require('fiber')
 [020] @@ -123,7 +123,7 @@
 [020]  ...
 [020]  s.index.sk:stat().memory.rows
 [020]  ---
 [020] -- 27
 [020] +- 23
 [020]  ...
 [020]  box.snapshot()
 [020]  ---

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat