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/object.kt

@@ -0,0 +1,7 @@
+fun box(): String {
+    object K {
+        val ok = "OK"
+    }
+
+    return K.ok
+}