[AIRFLOW-3885] Fix race condition in scheduler test

[astahlman]

Jira

• [ X ] 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-3885
  • 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

• [ X ] Here are some details about my PR, including screenshots of any UI changes:

This is a follow-up to #4730.

We're hitting this race condition frequently now that we don't sleep()
during unit tests. We don't actually need to assert that the task is
currently running - it's fine if it has already run successfully.

Tests

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

Commits

• [ X ] 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

• [ X ] Passes flake8

[astahlman]

cc @feng-tao I expect this to fix all of those failing builds on master

[codecov-io]

Codecov Report

Merging #4737 into master will increase coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4737      +/-   ##
==========================================
+ Coverage    74.5%   74.61%   +0.11%     
==========================================
  Files         430      430              
  Lines       28002    28002              
==========================================
+ Hits        20862    20893      +31     
+ Misses       7140     7109      -31

Impacted Files	Coverage Δ
airflow/jobs.py	76.46% <0%> (+0.71%)	⬆️
airflow/utils/dag_processing.py	59.89% <0%> (+1.06%)	⬆️
airflow/executors/__init__.py	63.46% <0%> (+3.84%)	⬆️
airflow/utils/sqlalchemy.py	81.81% <0%> (+4.54%)	⬆️
airflow/executors/sequential_executor.py	100% <0%> (+50%)	⬆️

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[feng-tao]

thanks @astahlman