backend/tests: add boxing{14,15}

the latter is disabled.

backend.native/tests/codegen/boxing/boxing14.kt

@@ -0,0 +1,5 @@
+fun main(args: Array<String>) {
+    42.println()
+}
+
+fun <T> T.println() = println(this.toString())

backend.native/tests/codegen/boxing/boxing15.kt

@@ -0,0 +1,5 @@
+fun main(args: Array<String>) {
+    println(foo(17))
+}
+
+fun <T : Int> foo(x: T): Int = x