A couple of new tests

for boxing
    and for body assignment in an interface

backend.native/tests/runtime/basic/interface0.kt

@@ -0,0 +1,17 @@
+
+interface A {
+    fun b() = c()
+    fun c()
+}
+
+class B(): A {
+    override fun c() {
+        println("PASSED")
+    }
+}
+
+fun main(args: Array<String>) {
+    val a:A = B()
+    a.b()
+}
+