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/kt285.kt

@@ -0,0 +1,12 @@
+trait Trait {
+    fun foo() = "O"
+    fun toString() = "K"
+}
+
+class SimpleClass : Trait
+
+class ComplexClass : Trait by SimpleClass() {
+    override fun toString() = foo() + super.toString()
+}
+
+fun box() = ComplexClass().toString()