test: flaky box/*.test.lua tests fails on very start

[avtikhon]

Tarantool version:
| Tarantool 2.7.0-73-g953986e5f
| 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

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/873752902#L4435

artifacts.zip

results file checksum: 72a91771dd0a3e4a1a4052f61e4b72e9

[012] box/ddl_collation.test.lua                                      [ fail ]
[012] 
[012] Test failed! Result content mismatch:
[012] --- box/ddl_collation.result	Fri Nov 13 15:27:44 2020
[012] +++ /tmp/tnt/rejects/box/ddl_collation.reject	Thu Nov 26 11:31:28 2020
[012] @@ -1,853 +1,4 @@
[012]  -- test-run result file version 2
[012] -env = require('test_run')
[012] - | ---
[012] - | ...

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat

[avtikhon]

Found that the issue happened when tests in parallel used all the memory available and it was not enough to start the new tests. Issue investigation provided at #98 .