add HiGHS to test_model.py (some still failing)

Author: rschwarz

test/test_model.py

@@ -7,6 +7,7 @@
     CBC,
     Column,
     GUROBI,
+    HIGHS,
     LinExpr,
     Model,
     MAXIMIZE,
@@ -19,7 +20,7 @@
 )
 
 TOL = 1e-4
-SOLVERS = [CBC]
+SOLVERS = [CBC, HIGHS]
 if "GUROBI_HOME" in os.environ:
     SOLVERS += [GUROBI]