backend/tests: Add blackbox tests from Kotlin JVM

Added tests from testData/codegen/box directory. There are blackbox tests
in other directories and they are to be added.

backend.native/tests/external/codegen/blackbox/classes/classObject.kt

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