Clean up numeric_expr dispatcher test driver

[jsiirola]

Fixes # .

Summary/Motivation:

This is a NFC PR that resolves a bug in the test driver for testing the numeric_expr dispatchers where one test could bleed into the next (specifically, when testing a mutable sum as the first argument, the conversion of that argument to immutable would bleed into the remaining tests in that test group). Fixing this (lines 207 and 263) led to 170 test failures because the baselines were referring to the mutable expressions. Updating the baselines to NOT refer tot he original mutable expressions resulted in identifying 4 tests whose baselines were "materially wrong" (i.e., the test was testing the wrong thing):

pyomo/core/tests/unit/test_numeric_expr_dispatcher.py::TestExpressionGeneration::test_sub_mutable_l0
pyomo/core/tests/unit/test_numeric_expr_dispatcher.py::TestExpressionGeneration::test_sub_mutable_l3
pyomo/core/tests/unit/test_numeric_expr_zerofilter.py::TestExpressionGeneration_ZeroFilter::test_sub_mutable_l0
pyomo/core/tests/unit/test_numeric_expr_zerofilter.py::TestExpressionGeneration_ZeroFilter::test_sub_mutable_l3

Note that this is only affecting test code: the expression system was - and continues to be - doing the right thing.

Changes proposed in this PR:

• Correct expression dispatcher test driver to not allow tests to bleed into each otehr

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1. I agree my contributions are submitted under the BSD license.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.70%. Comparing base (fa15cc7) to head (42735c3).
Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3551      +/-   ##
==========================================
- Coverage   88.70%   88.70%   -0.01%     
==========================================
  Files         888      888              
  Lines      102024   102024              
==========================================
- Hits        90505    90503       -2     
- Misses      11519    11521       +2     

Flag	Coverage Δ
builders	26.57% <ø> (+0.01%)	⬆️
default	84.82% <ø> (?)
expensive	33.96% <ø> (?)
linux	86.17% <ø> (-2.29%)	⬇️
linux_other	86.17% <ø> (-0.01%)	⬇️
osx	76.09% <ø> (-0.01%)	⬇️
win	84.66% <ø> (-0.01%)	⬇️
win_other	84.66% <ø> (-0.01%)	⬇️

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