backend/tests: Add blackbox tests from Kotlin JVM

Added tests from testData/codegen/box directory. There are blackbox tests
in other directories and they are to be added.

backend.native/tests/external/codegen/blackbox/increment/postfixIncrementOnClassSmartCast.kt

@@ -0,0 +1,11 @@
+open class Base
+class Derived: Base()
+operator fun Derived.inc(): Derived { return Derived() }
+
+public fun box() : String {
+    var i : Base
+    i = Derived()
+    val j = i++
+
+    return if (j is Derived && i is Derived) "OK" else "fail j = $j i = $i"
+}