test: flaky app/socket.test.lua test

[avtikhon]

Tarantool version:
Tarantool 2.5.0-27-g32f59756af
Target: Linux-x86_64-RelWithDebInfo
Build options: cmake . -DCMAKE_INSTALL_PREFIX=/usr/local -DENABLE_BACKTRACE=ON
Compiler: /usr/bin/clang-8 /usr/bin/clang++-8
C_FLAGS: -Wno-unknown-pragmas -fexceptions -funwind-tables -fno-omit-frame-pointer -fno-stack-protector -fno-common -msse2 -fsanitize=address -fsanitize-blacklist=/builds/pUzQszzd/0/tarantool/tarantool/asan/asan.supp -std=c11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Werror
CXX_FLAGS: -Wno-unknown-pragmas -fexceptions -funwind-tables -fno-omit-frame-pointer -fno-stack-protector -fno-common -msse2 -fsanitize=address -fsanitize-blacklist=/builds/pUzQszzd/0/tarantool/tarantool/asan/asan.supp -std=c++11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Werror

OS version:
Linux / OSX 15

Bug description:
OSX 15: https://gitlab.com/tarantool/tarantool/-/jobs/553612874
release ASAN + clang: https://gitlab.com/tarantool/tarantool/-/jobs/553555222

https://gitlab.com/tarantool/tarantool/-/jobs/784837412#L5248

artifacts.zip

results file checksum: 08dccf8bc443353e32999a98b3894589

 [054] --- app/socket.result	Tue May 12 15:51:33 2020
 [054] +++ app/socket.reject	Fri May 15 02:09:24 2020
 [054] @@ -752,7 +752,7 @@
 [054]  ...
 [054]  string.match(tostring(sc), ', peer') == nil
 [054]  ---
 [054] -- true
 [054] +- false
 [054]  ...
 [054]  sc:writable()
 [054]  ---
 [054] @@ -760,7 +760,7 @@
 [054]  ...
 [054]  string.match(tostring(sc), ', peer') == nil
 [054]  ---
 [054] -- true
 [054] +- false
 [054]  ...
 [054]  socket_error = sc:getsockopt('SOL_SOCKET', 'SO_ERROR')
 [054]  ---

Also found another issue in the test:

[043] --- app/socket.result     Thu Apr 30 08:50:16 2020
[043] +++ app/socket.reject     Fri May 15 05:20:09 2020
[043] @@ -1371,7 +1371,7 @@
[043]  ...
[043]  cnt
[043]  ---
[043] -- 2
[043] +- 3
[043]  ...
[043]  client:write('hi')
[043]  ---

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat