Enable parallel test-run

* `./test-run.py -j [JOBS]` runs tests simultaneously using up
  to JOBS working threads. If the -j option is given without an
  argument, test-run will use all available cores on your host.

* `./test-run.py` without ``-j` works as usual in sequential mode.

* In parallel mode `test/var/00N_XXX` directory contains logs and
  data files produced by [00N_XXX] thread, like var/ in sequential mode.

* `./test-run.py --reproduce ./test/reproduce/00N_XXX.yml` re-produces
  00N_XXX failed sequence.

Fixes https://github.com/tarantool/test-run/issues/7