test: flaky box/gh-2763-session-credentials-update.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.6.0-115-gabbd98dc8
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 15

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/760552969#L4396

artifacts.zip

results file checksum: 3caab61b50328a1b8bdaa0b2d857d95b

[056] --- box/gh-2763-session-credentials-update.result	Mon Sep 28 09:58:45 2020
[056] +++ box/gh-2763-session-credentials-update.reject	Mon Sep 28 13:59:04 2020
[056] @@ -54,6 +54,7 @@
[056]   | ...
[056]  box.schema.user.drop('test_user')
[056]   | ---
[056] + | - error: Unsupported role privilege '4'
[056]   | ...
[056]  
[056]  --
[056] @@ -148,12 +149,13 @@
[056]   | ...
[056]  box.schema.user.grant('guest', 'execute', 'universe')
[056]   | ---
[056] + | - error: User 'guest' is not found
[056]   | ...
[056]  -- Now the function owner's credentials should be updated, and
[056]  -- anyone called test3 should have updated rights.
[056]  box.func.test3:call()
[056]   | ---
[056] - | - success
[056] + | - error: Execute access to function 'test2' is denied for user 'guest'
[056]   | ...
[056]  
[056]  box.func.test3:drop()
[056] @@ -167,7 +169,9 @@
[056]   | ...
[056]  box.schema.user.revoke('guest', 'read, write', 'space', '_func')
[056]   | ---
[056] + | - error: User 'guest' does not have read, write access on space '_func'
[056]   | ...
[056]  box.schema.user.revoke('guest', 'execute', 'universe')
[056]   | ---
[056] + | - error: User 'guest' does not have execute access on universe ''
[056]   | ...
[056] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat