Test directory is polluted with `luatest.log` file

[locker]

Change directory to tarantool/test and run any test written in luatest, for example ./test-run.py box-luatest/watcher_test.lua, then check git status:

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        ../luatest.log
        luatest.log

The luatest.log file is also created and never deleted if a test is executed without test-run, for example luatest box-luatest/watcher_test.lua:

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        luatest.log

luatest must not create files in the test directory unless explicitly asked to.

Reproduced with https://github.com/tarantool/tarantool/commit/735b0dce76ff83a748e922b292580ebf48f2e946.

[d-enk]

even luatest -h creates a luatest.log file this is very strange behavior

--log /dev/null doesn't work