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

^KT-65979

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

@@ -0,0 +1,14 @@
+/*
+ * 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:Float, b:Int) = a + b
+
+fun box(): String {
+    assertEquals(3.0F, sum(1.0F, 2))
+    return "OK"
+}