tests: add unit test for add_constraint

[shadiakiki1986]

Tests for input nan, for lower-bound already set, and for upper-bound already set. This was prompted by PR 230 in Ipopt (link): Fix error in add_constraint NaN handling for EqualTo

I figured I'd add a unit test for the checks here, and they'd be useful for other solvers as well. I checked ECOS and SCS, but they don't implement add_constraint. OTOH, GLPK.jl does implement it (ref), but it needs this new test to be added to the unit test array (ref).

[odow]

So we already check for finiteness at the JuMP level: jump-dev/JuMP.jl#2272

I don't know if we want to make every solver check every coefficient for a NaN. That seems like a lot of overhead.

[odow]

Closing since there are already tests for LowerBoundAlreadySet:

MathOptInterface.jl/src/Test/modellike.jl

Lines 711 to 734 in 162049e

	function set_lower_bound_twice(model::MOI.ModelLike, T::Type)
	MOI.empty!(model)
	@test MOI.is_empty(model)
	x = MOI.add_variable(model)
	f = MOI.SingleVariable(x)
	lb = zero(T)
	@test MOI.supports_constraint(model, MOI.SingleVariable, MOI.GreaterThan{T})
	sets = [MOI.EqualTo(lb), MOI.Interval(lb, lb),
	MOI.Semicontinuous(lb, lb), MOI.Semiinteger(lb, lb)]
	set2 = MOI.GreaterThan(lb)
	for set1 in sets
	if !MOI.supports_constraint(model, MOI.SingleVariable, typeof(set1))
	continue
	end
	ci = MOI.add_constraint(model, f, set1)
	err = MOI.LowerBoundAlreadySet{typeof(set1), typeof(set2)}(x)
	@test_throws err MOI.add_constraint(model, f, set2)
	MOI.delete(model, ci)
	ci = MOI.add_constraint(model, f, set2)
	err = MOI.LowerBoundAlreadySet{typeof(set2), typeof(set1)}(x)
	@test_throws err MOI.add_constraint(model, f, set1)
	MOI.delete(model, ci)
	end
	end

and I don't think we need all solvers checking for isnan on every element. JuMP already does this, and either the lower-level solver should do it, or the person choosing to bypass JuMP and use MOI should do it.