[AIRFLOW-862] Fix Unit Tests for DaskExecutor

[jlowin]

Unit tests were inadvertently disabled for DaskExecutor added in #2067.

@bolkedebruin in reviewing the tests, I fixed the ImportError that caused them to be disabled, streamlined them slightly, and removed one that turned out to be useless without a BackfillJob checking the executor's event buffer -- that case is already covered by the Dask integration test.

Dear Airflow Maintainers,

Please accept this PR that addresses the following issues:

• https://issues.apache.org/jira/browse/AIRFLOW-862

[codecov-io]

Codecov Report

Merging #2076 into master will increase coverage by 0.31%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master    #2076      +/-   ##
==========================================
+ Coverage   66.67%   66.98%   +0.31%     
==========================================
  Files         142      142              
  Lines       10725    10724       -1     
==========================================
+ Hits         7151     7184      +33     
+ Misses       3574     3540      -34

Impacted Files	Coverage Δ
airflow/executors/dask_executor.py	81.39% <60%> (+81.39%)	✅
airflow/task_runner/bash_task_runner.py	93.33% <ø> (-6.67%)	❌
airflow/utils/helpers.py	41.08% <ø> (-2.48%)	❌
airflow/jobs.py	73.07% <ø> (ø)	✅
airflow/models.py	86.24% <ø> (+0.1%)	✅
airflow/executors/base_executor.py	94.64% <ø> (+3.57%)	✅

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[bolkedebruin]

@jlowin looks good, can you investigate why postgres is failing on py3? Maybe unrelated, not sure. and maybe rebase to see if decrease in coverage and landscape errors will go away.

[jlowin]

@bolke weird error... seems unrelated. I'm guessing the coverage decrease is because the unit test I deleted had a failed task in it and the remaining one doesn't. I don't really see what else could have caused that (the two unit tests were almost identical otherwise). I'm not sure what landscape's issue is either, when I click details it says no problems.

I'm going to rebase and push and let's see what happens!

[bolkedebruin]

LGTM