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/objects/kt2663.kt

@@ -0,0 +1,10 @@
+fun box() : String {
+    var a = 1
+
+    object {
+        init {
+            a = 2
+        }
+    }
+    return if (a == 2) "OK" else "fail"
+}