[Tests] Migrate backend-independent tests from native to compiler/testData.

^KT-65979

compiler/testData/codegen/box/function/extension.kt

@@ -0,0 +1,16 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the LICENSE file.
+ */
+// WITH_STDLIB
+
+import kotlin.test.*
+
+class B(val a: Int)
+
+fun B.foo() = this.a
+
+fun box(): String {
+    assertEquals(42, B(42).foo())
+    return "OK"
+}