Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/enum/innerWithExistingClassObject.kt

@@ -0,0 +1,8 @@
+class A {
+    class object {}
+    enum class E {
+        OK
+    }
+}
+
+fun box() = A.E.OK.toString()