unit test for colgen loop when infeasible master (Farkas pricing)

[guimarqu]

Artur's suggestion to address the "No variable" warning in the LpSolver.

[12:17]guimarqu: hey, how do you start column generation when there is no variable in the master ? This happens at first colgen iteration using Farkas pricing
[14:55]Artur Pessoa: To use the solver, I had to add a dummy variable with upper and lower bound equal to zero and disable preprocessing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.11%. Comparing base (e5e6c2b) to head (7a3818c).
⚠️ Report is 103 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1029   +/-   ##
=======================================
  Coverage   80.11%   80.11%           
=======================================
  Files          85       85           
  Lines        6428     6428           
=======================================
  Hits         5150     5150           
  Misses       1278     1278           

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