Migrate boxWithJava tests to multi-file framework

compiler/testData/codegen/boxWithJava/fileClasses/differentFiles.kt

@@ -0,0 +1,21 @@
+// FILE: Baz.java
+
+public class Baz {
+    public static String baz() {
+        return Foo.foo() + Bar.bar();
+    }
+}
+
+// FILE: bar.kt
+
+@file:JvmName("Bar")
+public fun bar(): String = "K"
+
+// FILE: foo.kt
+
+@file:JvmName("Foo")
+public fun foo(): String = "O"
+
+// FILE: test.kt
+
+fun box(): String = Baz.baz()