[AIRFLOW-3551] Improve BashOperator Test Coverage

[feluelle]

Make sure you have checked all steps below.

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-3551
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.

Description

• Here are some details about my PR, including screenshots of any UI changes:
• adds test case for xcom_push=True
• refactoring

Tests

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

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  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.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
  • All the public functions and the classes in the PR contain docstrings that explain what it does

Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #4367 into master will increase coverage by 1.29%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4367      +/-   ##
==========================================
+ Coverage   78.12%   79.42%   +1.29%     
==========================================
  Files         202      204       +2     
  Lines       16483    18586    +2103     
==========================================
+ Hits        12878    14762    +1884     
- Misses       3605     3824     +219

Impacted Files	Coverage Δ
airflow/operators/bash_operator.py	92.98% <100%> (+1.6%)	⬆️
airflow/utils/db.py	28.47% <0%> (-4.07%)	⬇️
airflow/models/dagpickle.py	94.11% <0%> (ø)
airflow/models/base.py	85.71% <0%> (ø)
airflow/models/__init__.py	94.35% <0%> (+1.67%)	⬆️
airflow/jobs.py	79.11% <0%> (+1.72%)	⬆️
airflow/utils/helpers.py	84.35% <0%> (+2.13%)	⬆️
airflow/bin/cli.py	66.72% <0%> (+2.24%)	⬆️
airflow/models/connection.py	93.44% <0%> (+4.83%)	⬆️

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[Fokko]

Restarted the failed job, to determine if the error is transient.

[kaxil]

Thanks @feluelle