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/kt3684.kt

@@ -0,0 +1,16 @@
+open class X(private val n: String) {
+
+    fun foo(): String {
+        return object : X("inner") {
+            fun print(): String {
+                return n;
+            }
+        }.print()
+    }
+}
+
+
+fun box() : String {
+  return X("OK").foo()
+}
+