[AIRFLOW-5468] Fix PubSub system test class name

[turbaszek]

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Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-5468

Description

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Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

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  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[codecov-io]

Codecov Report

Merging #6085 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6085      +/-   ##
==========================================
- Coverage   80.09%   80.08%   -0.01%     
==========================================
  Files         606      606              
  Lines       34893    34893              
==========================================
- Hits        27946    27945       -1     
- Misses       6947     6948       +1

Impacted Files	Coverage Δ
airflow/utils/dag_processing.py	58.8% <0%> (-0.19%)	⬇️

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