Apply suggestions from code review

Author: odow

test/MOI_wrapper.jl

@@ -482,6 +482,7 @@ function test_quadratic_modification_from_affine()
     x = MOI.add_variable(model)
     MOI.add_constraint(model, x, MOI.GreaterThan(2.0))
     f = 2.0 * x + 1.0
+    MOI.set(model, MOI.ObjectiveSense(), MOI.MIN_SENSE)
     attr = MOI.ObjectiveFunction{typeof(f)}()
     MOI.set(model, attr, f)
     MOI.optimize!(model)
@@ -500,6 +501,7 @@ function test_quadratic_off_diagonal_modification()
     x = MOI.add_variables(model, 2)
     MOI.add_constraint.(model, x, MOI.GreaterThan.([2.0, 3.0]))
     f = 4.0 * x[1] * x[1] + 2.0 * x[1] * x[2] + 2.0 * x[2] * x[2]
+    MOI.set(model, MOI.ObjectiveSense(), MOI.MIN_SENSE)
     attr = MOI.ObjectiveFunction{typeof(f)}()
     MOI.set(model, attr, f)
     MOI.optimize!(model)
@@ -524,6 +526,7 @@ function test_quadratic_diagonal_modification()
     x = MOI.add_variable(model)
     MOI.add_constraint(model, x, MOI.GreaterThan(2.0))
     f = 3.0 * x * x + 2.0 * x + 1.0
+    MOI.set(model, MOI.ObjectiveSense(), MOI.MIN_SENSE)
     attr = MOI.ObjectiveFunction{typeof(f)}()
     MOI.set(model, attr, f)
     MOI.optimize!(model)