Add [missing] test for integer variable

Refs. #233
Refs. jump-dev/MathOptInterface.jl#2517

examples/MINLPs/milp.jl

@@ -0,0 +1,8 @@
+function milp(; solver = nothing)
+    m = JuMP.Model(solver)
+
+    @variable(m, 0 <= objvar <= 20, Int)
+    @objective(m, Min, objvar)
+
+    return m
+end

test/test_algorithm.jl

@@ -1043,3 +1043,14 @@ end
     @test isapprox(objective_value(m), 13; atol = 1e-4)
     @test MOI.get(m, Alpine.NumberOfIterations()) == 0
 end
+
+@testset "Test integer variables" begin
+    test_solver = optimizer_with_attributes(
+        Alpine.Optimizer,
+        "nlp_solver" => IPOPT,
+        "mip_solver" => HIGHS,
+        "minlp_solver" => JUNIPER,
+    )
+    m = milp(solver = test_solver)
+    @test_throws "Alpine does not support MINLPs with generic integer (non-binary) variables yet!" JuMP.optimize!(m)
+end