[AIRFLOW-6467] Use self.dag i/o creating a new one

[mustafagok]

In this unit test method, self.dag can be used instead of creating a local variable named "dag".

Link to JIRA issue: https://issues.apache.org/jira/browse/AIRFLOW-6467

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[codecov-io]

Codecov Report

Merging #7067 into master will decrease coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7067      +/-   ##
==========================================
- Coverage   84.97%   84.72%   -0.26%     
==========================================
  Files         679      680       +1     
  Lines       38664    38748      +84     
==========================================
- Hits        32856    32830      -26     
- Misses       5808     5918     +110

Impacted Files	Coverage Δ
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	78.75% <0%> (-20%)	⬇️
airflow/example_dags/example_complex.py	100% <0%> (ø)

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