Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/functions/kt2716.kt

@@ -0,0 +1,11 @@
+package someTest
+
+public class Some private(val v: String) {
+    class object {
+        public fun init(v: String): Some {
+            return Some(v)
+        }
+    }
+}
+
+fun box() = Some.init("OK").v