test: flaky box/rtree_misc.test.lua test

[avtikhon]

Tarantool version:
| Tarantool 2.7.0-74-g6ad720d6d
| 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/874058328#L4978

artifacts.zip

results file checksum: af719d79ce29e0d15a1ab6449830c59f

[052] box/rtree_misc.test.lua                                         [ fail ]
[052] 
[052] Test failed! Result content mismatch:
[052] --- box/rtree_misc.result	Thu Nov 26 14:26:01 2020
[052] +++ /tmp/tnt/rejects/box/rtree_misc.reject	Thu Nov 26 15:07:07 2020
[052] @@ -171,10 +171,11 @@
[052]  -- and some records for chaos
[052]  for i = 1,10 do for j = 1,10 do s:insert{500+i+j*20, 500+i*20+j, {i, j, i, j}} end end
[052]  ---
[052] +- error: Duplicate key exists in unique index 'primary' in space 'spatial'
[052]  ...
[052]  s.index.spatial:count()
[052]  ---
[052] -- 600
[052] +- 529
[052]  ...
[052]  #s.index.spatial:select({3, 4, 5, 6})
[052]  ---
[052] @@ -185,7 +186,7 @@
[052]  ...
[052]  s.index.spatial:count()
[052]  ---
[052] -- 350
[052] +- 279
[052]  ...
[052]  #s.index.spatial:select({3, 4, 5, 6})
[052]  ---
[052] 

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat