Migrate boxAgainstJava tests to multi-file framework

compiler/testData/codegen/boxAgainstJava/sam/adapters/comparator.kt

@@ -1,3 +1,15 @@
+// FILE: JavaClass.java
+
+import java.util.*;
+
+class JavaClass {
+    public static void sortIntList(List<Integer> list, Comparator<Integer> comparator) {
+        Collections.sort(list, comparator);
+    }
+}
+
+// FILE: 1.kt
+
 import java.util.*
 
 fun box(): String {
@@ -5,4 +17,4 @@ fun box(): String {
     val expected = ArrayList(Arrays.asList(8, 5, 4, 3, 2, 1))
     JavaClass.sortIntList(list, { a, b -> b - a })
     return if (list == expected) "OK" else list.toString()
-}
+}