Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/extensionFunctions/kt475.kt

@@ -0,0 +1,18 @@
+import java.util.ArrayList
+
+fun box() : String {
+  val array = ArrayList<String>()
+  array.add("0")
+  array.add("1")
+  array.add("2")
+  array.last = "5"
+  return if(array.length == 3 && array.last == "5") "OK" else "fail"
+}
+
+var <T> ArrayList<T>.length : Int
+    get() = size()
+    set(value: Int) = throw java.lang.Error()
+
+var <T> ArrayList<T>.last : T
+    get() = get(size()-1)!!
+    set(el : T) { set(size()-1, el) }