test: flaky sql-tap/in2.test.lua test hangs

[avtikhon]

Tarantool version:
| Tarantool 2.7.0-74-g6ad720d6d6
| Target: Linux-x86_64-Debug
| Build options: cmake . -DCMAKE_INSTALL_PREFIX=/usr/local -DENABLE_BACKTRACE=ON
| Compiler: /usr/bin/cc /usr/bin/c++
| C_FLAGS: -fexceptions -funwind-tables -fno-omit-frame-pointer -fno-stack-protector -fno-common -fopenmp -msse2 -fprofile-arcs -ftest-coverage -std=c11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Wno-format-truncation -Wno-gnu-alignof-expression -fno-gnu89-inline -Wno-cast-function-type -Werror
| CXX_FLAGS: -fexceptions -funwind-tables -fno-omit-frame-pointer -fno-stack-protector -fno-common -fopenmp -msse2 -fprofile-arcs -ftest-coverage -std=c++11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Wno-format-truncation -Wno-invalid-offsetof -Wno-gnu-alignof-expression -Wno-cast-function-type -Werror

OS version:
Debian 9

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/875825783#L5193

artifacts.zip

results file checksum: d44ffefb3e395506fd92e149c3a53bd6

[038] sql-tap/in2.test.lua                            memtx           Test timeout of 170 secs reached	[ fail ]
[038] Test failed! Output from reject file var/rejects/sql-tap/in2.reject:
[038] TAP version 13
[038] 1..1999
[038] ok - in2-1
[038] ok - in2-2
...
[038] ok - in2-1832
[038] ok - in2-183
[038] Last 15 lines of Tarantool Log file [Instance "app_server"][/builds/WHBm2_Sk/0/tarantool/tarantool/test/var/038_sql-tap/in2.test.lua:memtx.tarantool.log]:
[038] Worker "038_sql-tap" got failed test; restarted the server

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat