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/reflection/classes/nestedClassesJava.kt

@@ -0,0 +1,26 @@
+// TODO: muted automatically, investigate should it be ran for JS or not
+// IGNORE_BACKEND: JS
+
+// WITH_REFLECT
+// FILE: J.java
+
+public class J {
+    public class Inner {}
+
+    public static class Nested {}
+
+    private static class PrivateNested {}
+
+    // This anonymous class should not appear in 'nestedClasses'
+    private final Object o = new Object() {};
+}
+
+// FILE: K.kt
+
+import kotlin.test.assertEquals
+
+fun box(): String {
+    assertEquals(listOf("Inner", "Nested", "PrivateNested"), J::class.nestedClasses.map { it.simpleName!! }.sorted())
+
+    return "OK"
+}