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/kt2288.kt

@@ -0,0 +1,8 @@
+public open class Test(): java.util.RandomAccess, Cloneable, java.io.Serializable
+{
+        public override fun clone(): Test = Test() // Override 'clone()' with more precise type 'Test'
+
+        public override fun toString() = "OK"
+}
+
+fun box() = Test().clone().toString()