Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/classes/kt707.kt

@@ -0,0 +1,9 @@
+class List<T>(val head: T, val tail: List<T>? = null)
+
+fun <T> List<T>.mapHead(f: (T)-> T): List<T> = List<T>(f(head), null)
+
+fun box() : String {
+  val a: Int = List<Int>(1).mapHead{it * 2}.head
+  System.out?.println(a)
+  return "OK"
+}