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/kt508.kt

@@ -0,0 +1,17 @@
+package mult_constructors_3_bug
+
+public open class Identifier() {
+    private var myNullable : Boolean = true
+    class object {
+        open public fun init(isNullable : Boolean) : Identifier {
+            val __ = Identifier()
+            __.myNullable = isNullable
+            return __
+        }
+    }
+}
+
+fun box() : String {
+    Identifier.init(true)
+    return "OK"
+}