Migrate boxWithJava tests to multi-file framework

compiler/testData/codegen/boxWithJava/innerClass.kt

@@ -0,0 +1,21 @@
+// FILE: JavaClass.java
+
+public abstract class JavaClass {
+    public abstract InnerClass onCreateInner();
+
+    public class InnerClass {
+
+    }
+}
+
+// FILE: Kotlin.kt
+
+public class MyWallpaperService : JavaClass() {
+    override fun onCreateInner(): JavaClass.InnerClass = MyEngine()
+
+    private inner class MyEngine : JavaClass.InnerClass()
+}
+
+fun box(): String {
+    return if (MyWallpaperService().onCreateInner() != null) return "OK" else "fail"
+}