backend/tests: add boxing{14,15}

the latter is disabled.

backend.native/tests/build.gradle

@@ -502,6 +502,17 @@ task boxing13(type: RunKonanTest) {
     source = "codegen/boxing/boxing13.kt"
 }
 
+task boxing14(type: RunKonanTest) {
+    goldValue = "42\n"
+    source = "codegen/boxing/boxing14.kt"
+}
+
+task boxing15(type: RunKonanTest) {
+    disabled = true
+    goldValue = "17\n"
+    source = "codegen/boxing/boxing15.kt"
+}
+
 task interface0(type: RunKonanTest) {
     goldValue = "PASSED\n"
     source = "runtime/basic/interface0.kt"

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