[AIRFLOW-5698][part of AIRFLOW-5697] Organize Dataflow tests

[mik-laj]

This PR is one of a series that aims to improve this integration
https://issues.apache.org/jira/browse/AIRFLOW-5697

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[jaketf]

@tvalentyn are there any specific limitations on the first character of a dataflow job id ? (e.g. can't be _ or -. If so, I think that the hook might cast _ -> - and think it has fixed the name but in reality this may still be invalid. we should add a test case for something like this.

[mik-laj]

The name must match the regular expression a-z?

I like the idea of trying to fix IDs. I enter it in the list of planned improvements. Thanks.

[tvalentyn]

Missed this question, dataflow job IDs start with a date, e.g. 2019-08-05