Migrate boxWithJava tests to multi-file framework

compiler/testData/codegen/boxWithJava/mangling.kt

@@ -0,0 +1,21 @@
+// FILE: JavaClass.java
+
+public class JavaClass extends A {
+    public String test() {
+        return "Java";
+    }
+}
+
+// FILE: test.kt
+
+open class A {
+    internal open fun test(): String = "Kotlin"
+}
+
+fun box(): String {
+    if (A().test() != "Kotlin") return "fail 1: ${A().test()}"
+
+    if (JavaClass().test() != "Java") return "fail 2: ${JavaClass().test()}"
+
+    return "OK"
+}