GAMS: fix pickling models, update tests

[jsiirola]

Fixes # .

Summary/Motivation:

This resolves some long-standing issues in the GAMS interface that were not (until recently) being caught by the test harness

Changes proposed in this PR:

• Implement custom picklable GAMSSymbolMap
• Update list of expected GAMS test failures

Legal Acknowledgement

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

Looks good minus one question

[codecov]

Codecov Report

Patch coverage: 87.50% and no project coverage change.

Comparison is base (be93e57) 87.82% compared to head (777d242) 87.82%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2913   +/-   ##
=======================================
  Coverage   87.82%   87.82%           
=======================================
  Files         770      770           
  Lines       89632    89637    +5     
=======================================
+ Hits        78721    78726    +5     
  Misses      10911    10911           

Flag	Coverage Δ
linux	84.89% <87.50%> (+0.03%)	⬆️
osx	74.63% <87.50%> (+0.04%)	⬆️
other	85.07% <87.50%> (+0.03%)	⬆️
win	82.14% <87.50%> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
pyomo/repn/plugins/gams_writer.py	93.21% <87.50%> (+0.08%)	⬆️

... and 1 file with indirect coverage changes

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