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/kt1726.kt

@@ -0,0 +1,15 @@
+class Foo(
+        var state : Int,
+        val f : (Int) -> Int){
+
+    fun next() : Int {
+        val nextState = f(state)
+        state = nextState
+        return state
+    }
+}
+
+fun box(): String {
+    val f = Foo(23, {x -> 2 * x})
+    return if (f.next() == 46) "OK" else "fail"
+}