json: increase test cases before enabling sumtype decode in all json libraries (#21958)

vlib/json/json_sumtype_test.v

@@ -14,6 +14,16 @@ struct Price {
 	net f64
 }
 
+type Animal = Cat | Dog
+
+struct Cat {
+	cat_name string
+}
+
+struct Dog {
+	dog_name string
+}
+
 fn test_main() {
 	data := '{"attributes": {"price": [{"net": 1, "_type": "Price"}, {"net": 2, "_type": "Price"}]}}'
 	entity := json.decode(ShopResponseData, data) or { panic(err) }
@@ -55,3 +65,43 @@ fn test_main() {
 		}
 	}
 }
+
+fn test_sum_types() {
+	data1 := json.encode(Animal(Dog{
+		dog_name: 'Caramelo'
+	}))
+	assert data1 == '{"dog_name":"Caramelo","_type":"Dog"}'
+
+	s := '{"_type":"Cat","cat_name":"Whiskers"}'
+	animal := json.decode(Animal, s) or {
+		println(err)
+		assert false
+		return
+	}
+
+	assert animal is Cat
+	if animal is Cat {
+		assert animal.cat_name == 'Whiskers'
+	} else {
+		assert false, 'Wrong sumtype decode. In this case animal is a Cat'
+	}
+
+	s2 := '[{"_type":"Cat","cat_name":"Whiskers"}, {"_type":"Dog","dog_name":"Goofie"}]'
+
+	animals := json.decode([]Animal, s2) or {
+		println(err)
+		assert false
+		return
+	}
+
+	assert animals.len == 2
+	assert animals[0] is Cat
+	assert animals[1] is Dog
+	cat := animals[0] as Cat
+	dog := animals[1] as Dog
+	assert cat.cat_name == 'Whiskers'
+	assert dog.dog_name == 'Goofie'
+
+	j := json.encode(animals[0])
+	assert j == '{"cat_name":"Whiskers","_type":"Cat"}'
+}