[AIRFLOW-5699][part of AIRFLOW-5697][depends on AIRFLOW-5698] Add more tests for Dataflow integration

[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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Jira

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Tests

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@b8c0263). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #6369   +/-   ##
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  Coverage          ?   80.12%           
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  Files             ?      616           
  Lines             ?    35807           
  Branches          ?        0           
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  Hits              ?    28690           
  Misses            ?     7117           
  Partials          ?        0

Impacted Files	Coverage Δ
airflow/gcp/hooks/dataflow.py	91.07% <100%> (ø)

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