Migrate boxAgainstJava tests to multi-file framework

compiler/testData/codegen/boxAgainstJava/sam/differentFqNames.kt

@@ -1,7 +1,17 @@
+// FILE: Custom.java
+
+class Custom {
+    public interface Runnable {
+        void run2();
+    }
+}
+
+// FILE: 1.kt
+
 fun box(): String {
     val f = { }
     val class1 = Runnable(f).javaClass
     val class2 = Custom.Runnable(f).javaClass
 
     return if (class1 != class2) "OK" else "Same class: $class1"
-}
+}