Regression tests contain select(nil)

[ligurio]

When luatest running with -c (no capture) logs contains many warnings about using select() without limitations:

2022-07-12 21:10:14.225 [6437] main/103/luatest I> set 'log_format' configuration option to "plain"
2022-07-12 21:10:14.231 [6437] main/103/luatest C> Potentially long select from space '_ddl_sharding_key' (512)                     stack traceback:                                                                                                                  
        builtin/box/schema.lua:2382: in function 'check_select_safety'
        builtin/box/schema.lua:2396: in function 'select'                                                                          
        ...geyb/sources/MRG/ddl/test/set_sharding_metadata_test.lua:180: in function 'method'                   
        ...ources/MRG/ddl/.rocks/share/tarantool/luatest/runner.lua:348: in function <...ources/MRG/ddl/.rocks/share/tarantool/luatest/runner.lua:347>    

Version: ac006ff (tag 1.6.1)