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/enum.kt

@@ -0,0 +1,8 @@
+fun box(): String {
+    enum class K {
+        O
+        K
+    }
+
+    return K.O.toString() + K.K.toString()
+}