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/innerNested/nestedClassObject.kt

@@ -0,0 +1,12 @@
+class Outer {
+    class Nested {
+        companion object {
+            val O = "O"
+            val K = "K"
+        }
+    }
+    
+    fun O() = Nested.O
+}
+
+fun box() = Outer().O() + Outer.Nested.K