Improve system tests for Cloud Build

[mik-laj]

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Issue link: WILL BE INSERTED BY boring-cyborg

Make sure to mark the boxes below before creating PR: [x]

• Description above provides context of the change
• Unit tests coverage for changes (not needed for documentation changes)
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as explained in Contribution Workflow Example.

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In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.
Read the Pull Request Guidelines for more information.

[codecov-io]

Codecov Report

Merging #8003 into master will decrease coverage by 0.75%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #8003      +/-   ##
==========================================
- Coverage   87.16%   86.41%   -0.76%     
==========================================
  Files         931      932       +1     
  Lines       45172    45187      +15     
==========================================
- Hits        39375    39049     -326     
- Misses       5797     6138     +341     

Impacted Files	Coverage Δ
...s/google/cloud/example_dags/example_cloud_build.py	100.00% <100.00%> (ø)
...flow/providers/apache/cassandra/hooks/cassandra.py	21.25% <0.00%> (-72.50%)	⬇️
...w/providers/apache/hive/operators/mysql_to_hive.py	35.84% <0.00%> (-64.16%)	⬇️
airflow/kubernetes/volume_mount.py	44.44% <0.00%> (-55.56%)	⬇️
airflow/providers/postgres/operators/postgres.py	47.82% <0.00%> (-52.18%)	⬇️
airflow/providers/redis/operators/redis_publish.py	50.00% <0.00%> (-50.00%)	⬇️
airflow/kubernetes/volume.py	52.94% <0.00%> (-47.06%)	⬇️
airflow/providers/mongo/sensors/mongo.py	53.33% <0.00%> (-46.67%)	⬇️
airflow/kubernetes/pod_launcher.py	47.18% <0.00%> (-45.08%)	⬇️
airflow/providers/mysql/operators/mysql.py	55.00% <0.00%> (-45.00%)	⬇️
... and 11 more

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