Migrate boxWithJava tests to multi-file framework

compiler/testData/codegen/boxWithJava/reflection/nestedClasses.kt

@@ -0,0 +1,22 @@
+// 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"
+}