test: flaky box/net.box_field_names_gh-2978.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.7.0-43-gf9b515406
Target: Darwin-x86_64-RelWithDebInfo
Build options: cmake . -DCMAKE_INSTALL_PREFIX=/Users/tntmac03.tarantool.i/builds/6KMaH8fQ/0/tarantool/tarantool/static-build/tarantool-prefix -DENABLE_BACKTRACE=TRUE
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/844423513#L5735

artifacts.zip

results file checksum: 1371cceed747c5a4875ba8ff7047fa3a

[010] --- box/net.box_field_names_gh-2978.result	Fri Nov  6 11:35:25 2020
[010] +++ /tmp/tnt/rejects/box/net.box_field_names_gh-2978.reject	Thu Nov 12 04:57:41 2020
[010] @@ -98,4 +98,5 @@
[010]  ...
[010]  box.schema.user.revoke('guest', 'read, write, execute', 'space')
[010]  ---
[010] +- error: User 'guest' does not have read, write, execute access on space ''
[010]  ...
[010] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat