Fix typo in FBBT (GeneralExpression)

[jsiirola]

Fixes #2846

Summary/Motivation:

This fixes a typo in FBBT, and adds a test to exercise the code.

Changes proposed in this PR:

• Fix typo, add test

Legal Acknowledgement

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

Thanks!

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (d8cbd46) 87.44% compared to head (c2fe429) 87.44%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2848   +/-   ##
=======================================
  Coverage   87.44%   87.44%           
=======================================
  Files         771      771           
  Lines       89539    89539           
=======================================
+ Hits        78294    78296    +2     
+ Misses      11245    11243    -2     

Flag	Coverage Δ
linux	84.50% <100.00%> (+<0.01%)	⬆️
osx	74.54% <100.00%> (+<0.01%)	⬆️
other	84.68% <100.00%> (+<0.01%)	⬆️
win	82.08% <100.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
pyomo/contrib/fbbt/fbbt.py	96.96% <100.00%> (+1.06%)	⬆️

... and 4 files with indirect coverage changes

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