Remove ignore tag for tests that don't fail any more

Check for execution correctness inside the test
Fix run_external to include compileKotlinAgainstKotlin

backend.native/tests/external/codegen/box/classes/kt1157.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: NATIVE
 public object SomeClass {
     private val work = object : Runnable {
         override fun run() {

backend.native/tests/external/codegen/box/classes/privateOuterFunctions.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: NATIVE
 class C {
     private fun String.ext() : String = ""
     private fun f() {}

backend.native/tests/external/codegen/box/classes/privateOuterProperty.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: NATIVE
 class C{
     private var v : Int = 0