Feature/cucumber junit report

[janluak]

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Hi pytest-bdd team,

thanks for the great work! Totally in love with BDD and awesome it works out of the box with pytest 🎉

At our project needed a junit formatted report for the output of the bdd tests (gitlab isn't providing a native support for other pytest-bdd reports). Since I didn't like the idea of exporting the cucumberjson and then using a third party to convert to junit-xml I simply added the support natively to pytest-bdd with this PR. Additionally, I fixed https://github.com/pytest-dev/pytest-bdd/issues/567 (at least for steps, not a full feature) with it.

I am not really happy with the implementation of cucumber reports → if you approve the first version here I'd provide a rewrite of the reporting which is a bit cleaner and easier to adopt the additional feature list of cucumber reports. E.g. I'd not use the json format to generate the xml - afaik the different reports have different capabilities which doesn't help converting them from one to another when having the source data available...

Anyways, happy to get feedback ;)