Add a test demonstrating folding order

test/list/reduce.wren

@@ -1,4 +1,5 @@
 var a = [1, 4, 2, 1, 5]
+var b = ["W", "o", "r", "l", "d"]
 var max = new Fn {|a, b| a > b ? a : b }
 var sum = new Fn {|a, b| a + b }
 
@@ -7,3 +8,7 @@ IO.print(a.reduce(10, max)) // expect: 10
 
 IO.print(a.reduce(sum)) // expect: 13
 IO.print(a.reduce(-1, sum)) // expect: 12
+
+// sum also concatenates strings
+IO.print(b.reduce("Hello ", sum)) // expect: Hello World
+IO.print(b.reduce(sum)) // expect: World