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/postfixNullableClassIncrement.kt

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