[AIRFLOW-4058] Rename models tests

[ffinfo]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-4058

Description

Renaming models.py to test_models.py

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
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #4901 into master will increase coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4901      +/-   ##
==========================================
+ Coverage    75.3%   75.53%   +0.22%     
==========================================
  Files         450      450              
  Lines       29034    29034              
==========================================
+ Hits        21865    21931      +66     
+ Misses       7169     7103      -66

Impacted Files	Coverage Δ
airflow/contrib/operators/ssh_operator.py	83.54% <0%> (+1.26%)	⬆️
airflow/hooks/dbapi_hook.py	87.9% <0%> (+8.87%)	⬆️
airflow/utils/db.py	90.38% <0%> (+51.92%)	⬆️

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[ashb]

@ffinfo It seems that since we merged this commit something has happened to our logging config in tests, and we now see everything at debug level in the logs (example https://travis-ci.org/apache/airflow/jobs/504637817 compared to the build on master just before https://travis-ci.org/apache/airflow/jobs/504628389) which makes it even harder to find failures :(

I can't see anything in the diff that might cause this, but maybe there was some side-effect from somewhere else? Would you have a spare hour to try and look at what is causing it?

[ffinfo]

@ashb I did do some tracing but I think the cause is not in this merge request.

I checked my own fork on travis. Here on the same commit it did not happen:
https://travis-ci.org/ffinfo/airflow/jobs/504557217

The only difference between the build is that travis did a auto rebase on master before build, see also:
0fd8da0
(I did not generate this commit, travis does this automated.

This means that between my latest rebase/merge and the point where I did create the PR there was an other change on master that did caused this.

I will try to back trace this to the commit on master that did cause this. And make a PR to fix this.

[ashb]

Thanks for taking the time! @XD-DENG tracked it to this commit on master, but we couldn't see anything obvious that might cause it either.