test: flaky app/fiber.test.lua test storage.key null

[avtikhon]

Tarantool version:
Tarantool 2.5.1-92-ge2f2daea2
Target: Linux-x86_64-RelWithDebInfo
Build options: cmake . -DCMAKE_INSTALL_PREFIX=/usr -DENABLE_BACKTRACE=OFF
Compiler: /usr/bin/cc /usr/bin/c++
C_FLAGS:-O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fexceptions -funwind-tables -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 -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fexceptions -funwind-tables -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:
Linux (CentOS 8)

Bug description:
Found the issue one line above the issue:
"test: flaky app/fiber.test.lua test" #220

https://gitlab.com/tarantool/tarantool/-/jobs/758325296#L2984

artifacts.zip

results file checksum: fe10aeb6841cae7f47c5e3bb8256dee9

[175] --- app/fiber.result	Fri Sep 25 19:52:17 2020
[175] +++ app/fiber.reject	Fri Sep 25 19:58:21 2020
[175] @@ -670,7 +670,7 @@
[175]  ...
[175]  fiber.self().storage.key -- our local storage is not affected by f
[175]  ---
[175] -- 48
[175] +- null
[175]  ...
[175]  -- attempt to access local storage of dead fiber raises error
[175]  pcall(function(f) return f.storage end, f)
[175] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat