Migrate boxAgainstJava tests to multi-file framework

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

@@ -1,5 +1,15 @@
+// FILE: JavaClass.java
+
+class JavaClass {
+    public static void run(Runnable r) {
+        r.run();
+    }
+}
+
+// FILE: 1.kt
+
 fun box(): String {
     var v = "FAIL"
     JavaClass.run { v = "OK" }
     return v
-}
+}