test: flaky box/alter.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.7.0-71-gbf74a97fe
Target: Darwin-x86_64-RelWithDebInfo
Build options: cmake . -DCMAKE_INSTALL_PREFIX=/Users/tntmac04.tarantool.i/builds/PVWCJs8U/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/865123485#L6128

artifacts.zip

results file checksum: 26e862f898badb0fd45c429ace27cd92

[044] --- box/alter.result	Fri Nov 20 23:34:49 2020
[044] +++ /tmp/tnt/rejects/box/alter.reject	Sun Nov 22 07:06:34 2020
[044] @@ -160,9 +160,11 @@
[044]  -- uniformn weather access_sysview.test is completed of not.
[044]  box.space._space.index.owner:alter{parts = {2, 'unsigned'}}
[044]  ---
[044] +- error: 'builtin/box/schema.lua:1297: attempt to index local ''tuple'' (a nil value)'
[044]  ...
[044]  box.space._vspace.index.owner:alter{parts = {2, 'unsigned'}}
[044]  ---
[044] +- error: 'builtin/box/schema.lua:1297: attempt to index local ''tuple'' (a nil value)'
[044]  ...
[044]  _index:select{}
[044]  ---
[044] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat