[K/N][Tests] Adjust moved codegen tests to new infra

^KT-61259

native/native.tests/testData/codegen/function/extension.kt

@@ -3,6 +3,13 @@
  * that can be found in the LICENSE file.
  */
 
+import kotlin.test.*
+
 class B(val a: Int)
 
-fun B.foo() = this.a
+fun B.foo() = this.a
+
+fun box(): String {
+    assertEquals(42, B(42).foo())
+    return "OK"
+}