Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/localClasses/withclosure.kt

@@ -0,0 +1,8 @@
+fun box(): String {
+    val x = "OK"
+    class Aaa {
+        val y = x
+    }
+
+    return Aaa().y
+}