Migrate boxAgainstJava tests to multi-file framework

compiler/testData/codegen/boxAgainstJava/staticFun/classWithNestedEnum.kt

@@ -1,5 +1,17 @@
+// FILE: test/JavaClass.java
+
+package test;
+
+public class JavaClass {
+    public enum E { ENTRY }
+    
+    public static String foo() { return "OK"; }
+}
+
+// FILE: 1.kt
+
 package test
 
 fun box(): String {
-    return classWithNestedEnum.foo()!!
+    return JavaClass.foo()!!
 }