xpress_direct: resolve error retrieving reduced costs

[jsiirola]

Fixes # .

Summary/Motivation:

Pyomo/mpi-sppy#451 pointed out a typo in the xpress_direct interface that prevented retrieving the reduced costs. This resolves that error and adds tests to exercise that functionality.

Changes proposed in this PR:

• xpress_direct: fix typo (calling self._getRedCosts)
• xpress_direct: add test for retrieving rc and slack Suffixes

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.72%. Comparing base (180924a) to head (03ec9d2).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3422      +/-   ##
==========================================
+ Coverage   88.69%   88.72%   +0.03%     
==========================================
  Files         882      882              
  Lines      100615   100615              
==========================================
+ Hits        89237    89268      +31     
+ Misses      11378    11347      -31     

Flag	Coverage Δ
linux	86.18% <100.00%> (+0.03%)	⬆️
osx	76.20% <100.00%> (+0.02%)	⬆️
other	86.68% <100.00%> (+0.03%)	⬆️
win	84.56% <100.00%> (+0.02%)	⬆️

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