test: flaky engine/errinj_ddl.test.lua test hangs

[avtikhon]

Tarantool version:
| Tarantool 2.7.0-74-g6ad720d6d
| 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 -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 -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:
Linux

Bug description:
https://gitlab.com/tarantool/tarantool/-/jobs/876110769#L4797

results file checksum: 294f6d8da54e492470eb916b862a6dbb

[072] engine/errinj_ddl.test.lua                      vinyl           Test timeout of 170 secs reached	[ fail ]
[072] 
[072] Test failed! Result content mismatch:
[072] --- engine/errinj_ddl.result	Fri Nov 27 06:03:53 2020
[072] +++ /rw_bins/test/var/rejects/engine/errinj_ddl.reject	Fri Nov 27 06:08:14 2020
[072] @@ -64,441 +64,3 @@
[072]  - ok
[072]  ...
[072]  s:format(format) -- must fail
[072] ----
[072] -- error: 'Tuple field 2 type does not match one required by operation: expected string'
[072] -...

Steps to reproduce:

Optional (but very desirable):

• coredump
• backtrace
• netstat