Make sure top-level model is not garbage collected in UnitTestHarness

[bknueven]

Fixes/Resolves: N/A

Fixes failures on #1301.

Summary/Motivation:

The UnitTestHarness was implemented in such a fashion that the top-level Pyomo model could be garbage collected while it was running, leading to unpredictable behavior. This unpredictable behavior can be made deterministic by manually invoking Python's garbage collector (409e529).

This PR would make a few changes to the UnitTestHarness API to ensure the top-level Pyomo model is not garbage collected during tests. It also updates the UnitTestHarness to use the IDAES initialization_tester function, as this was now an easy fix.

Changes proposed in this PR:

• Demonstrate problem (409e529)
• Return top-level model from configure (9f31b1d)
• Use IDAES initialization_tester (52e18eb)
• Add top-level model to frame to ensure its not GC'ed (f93d5c1)
• Update documentation (761cd02)
• Cleanup from 409e529 (f9eff1b)
• Make UnitTestHarness a proper abstract base class (ba7d92f)

Legal Acknowledgement

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1. I agree my contributions are submitted under the license terms described in the LICENSE.txt file at the top level of this directory.
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 94.39%. Comparing base (d035b7e) to head (f05bd50).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1317   +/-   ##
=======================================
  Coverage   94.39%   94.39%           
=======================================
  Files         371      371           
  Lines       37922    37922           
=======================================
  Hits        35796    35796           
  Misses       2126     2126           

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[MarcusHolly]

LGTM - thanks for handling this