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/selfcreate.kt

@@ -0,0 +1,10 @@
+class B () {}
+
+open class A(val b : B) {
+    fun a() = object: A(b) {}
+}
+
+fun box() : String {
+    A(B()).a()
+    return "OK"
+}