Set maxDiff=None on the base TestCase class

[jsiirola]

Fixes # .

Summary/Motivation:

By default, unittest assertions (severely) truncate the diff error message when a comparison test fails. As we (almost?) always want to see the full diff in order to diagnose the test failure, we end up sprinkling self.maxDiff = None throughout the codebase. This PR adds the flag to the base TestCase class.

Changes proposed in this PR:

• set maxDiff = None in pyomo.common.unittest.TestCase

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.38%. Comparing base (26e4970) to head (e46d2b1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3171   +/-   ##
=======================================
  Coverage   88.38%   88.38%           
=======================================
  Files         846      846           
  Lines       94857    94858    +1     
=======================================
+ Hits        83835    83844    +9     
+ Misses      11022    11014    -8     

Flag	Coverage Δ
linux	86.33% <100.00%> (+<0.01%)	⬆️
osx	75.73% <100.00%> (+<0.01%)	⬆️
other	86.53% <100.00%> (+<0.01%)	⬆️
win	83.82% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.