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/extensionFunctions/virtual.kt

@@ -0,0 +1,24 @@
+class Request(val path: String) {
+
+}
+
+class Handler() {
+    fun Int.times(op: ()-> Unit) {
+        for(i in 0..this)
+            op()
+    }
+
+//    fun Request.getPath() : String {
+//        val sb = java.lang.StringBuilder()
+//        10.times {
+//            sb.append(path)?.append(this)
+//        }
+//        return sb.toString() as String
+//    }
+
+    fun Request.getPath() = path
+
+    fun test(request: Request) = request.getPath()
+}
+
+fun box() : String = if(Handler().test(Request("239")) == "239") "OK" else "fail"