backend/tests: add defaults{4,5,6}

backend.native/tests/codegen/function/defaults5.kt

@@ -0,0 +1,14 @@
+class Test(val x: Int) {
+    fun foo(y: Int = x) {
+        println(y)
+    }
+}
+
+fun Test.bar(y: Int = x) {
+    println(y)
+}
+
+fun main(args: Array<String>) {
+    Test(5).foo()
+    Test(6).bar()
+}