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/classObject.kt

@@ -0,0 +1,11 @@
+class C() {
+  class object {
+    fun create() = C()
+  }
+}
+
+fun box(): String {
+  val c = C.create()
+  return if (c is C) "OK" else "fail"
+}
+