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/enum/abstractNestedClass.kt

@@ -0,0 +1,10 @@
+enum class E {
+    ENTRY;
+
+    abstract class Nested
+}
+
+fun box(): String {
+    E.ENTRY
+    return "OK"
+}