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/when/emptyWhen.kt

@@ -0,0 +1,7 @@
+enum class A { X1, X2 }
+
+fun box(): String {
+    when {}
+    when (A.X1) {}
+    return "OK"
+}