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/traits/traitWithPrivateMember.kt

@@ -0,0 +1,26 @@
+interface Z {
+
+    fun testFun() : String {
+        return privateFun()
+    }
+
+    fun testProperty() : String {
+        return privateProp
+    }
+
+    private fun privateFun(): String {
+        return "O"
+    }
+
+    private val privateProp: String
+        get() = "K"
+}
+
+object Z2 : Z {
+
+}
+
+fun box() : String {
+    return Z2.testFun() + Z2.testProperty()
+}
+