Semi-Fix for #18

It fixes the output, but there are still issues, highlighted in `example/gherkin/backgrounds_and_line_comments.feature`.

We need each scenario to have its own background ran right before him.
Right now, they're ran too soon.

And without usage of the worker, even less steps pass.

Ideas ?

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This feels like wandering in a maze of Futures. Enough for today for me.