Improve tests for Storing Dag Code in DB

[kaxil]

The original test does not cover the real case where DAG model actually calls dag.sync_to_db(). This is where the subdag fileloc is set to parent dag's fileloc, so it is important to change this behavior

Based on #8151

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[codecov-io]

Codecov Report

Merging #8152 into master will increase coverage by 21.64%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #8152       +/-   ##
===========================================
+ Coverage   66.39%   88.03%   +21.64%     
===========================================
  Files         935      935               
  Lines       45170    45170               
===========================================
+ Hits        29990    39766     +9776     
+ Misses      15180     5404     -9776     

Impacted Files	Coverage Δ
airflow/kubernetes/volume_mount.py	44.44% <0.00%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0.00%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	47.18% <0.00%> (-39.44%)	⬇️
...viders/cncf/kubernetes/operators/kubernetes_pod.py	69.69% <0.00%> (-25.26%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0.00%> (-23.53%)	⬇️
airflow/providers/google/cloud/operators/vision.py	99.63% <0.00%> (+0.36%)	⬆️
...osoft/azure/operators/azure_container_instances.py	78.28% <0.00%> (+0.65%)	⬆️
airflow/cli/cli_parser.py	97.24% <0.00%> (+0.91%)	⬆️
airflow/providers/postgres/hooks/postgres.py	94.36% <0.00%> (+1.40%)	⬆️
...rflow/providers/databricks/operators/databricks.py	92.24% <0.00%> (+1.72%)	⬆️
... and 498 more

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[kaxil]

The PR targetting v1-10-test has been approved and merged