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

^KT-65979

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

@@ -0,0 +1,18 @@
+/*
+ * 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.*
+
+fun sum(a:Int, b:Int):Int {
+ var c:Int
+ c = a + b
+ return c
+}
+
+fun box(): String {
+  assertEquals(50, sum(42, 8))
+  return "OK"
+}