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/sealed/objects.kt

@@ -0,0 +1,11 @@
+sealed class Season {
+    object Warm: Season()
+    object Cold: Season()
+}
+
+fun foo(): Season = Season.Warm
+
+fun box() = when(foo()) {
+    Season.Warm -> "OK"
+    Season.Cold -> "Fail: Cold, should be Warm"
+}