Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/closures/extensionClosure.kt

@@ -0,0 +1,13 @@
+class Point(val x : Int, val y : Int)
+
+fun box() : String {
+    val answer = apply(Point(3, 5), { Point.(scalar : Int) : Point ->
+        Point(x * scalar, y * scalar)
+    })
+
+    return if (answer.x == 6 && answer.y == 10) "OK" else "FAIL"
+}
+
+fun apply(arg:Point, f :  Point.(scalar : Int) -> Point) : Point {
+    return arg.f(2)
+}