Tests doesn't cleanup working directories

Temporary working directories created and cleaned inside tarantool session.
This causes tarantool to recreate the working directory on exit.
Files left after running one test:
``` sh
$ find /tmp -regextype posix-extended -regex '/tmp/[a-zA-Z0-9]{6}' | xargs ls
/tmp/Z01zrj:
00000000000000000019.xlog

/tmp/jbyOjy:
00000000000000000019.xlog
```