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/nested2.kt

@@ -0,0 +1,8 @@
+fun box() : String {
+  val y = 12
+  val op = { (x:Int) -> (x + y).toString() }
+
+  val op2 : Int.(Int) -> String = { op(this + it) }
+
+  return if("27" == 5.op2(10)) "OK" else "fail"
+}