Migrate boxAgainstJava tests to multi-file framework

compiler/testData/codegen/boxAgainstJava/notNullAssertions/Delegation.java

@@ -1,7 +0,0 @@
-public class Delegation {
-    public static class ReturnNull {
-        public String foo() {
-            return null;
-        }
-    }
-}

compiler/testData/codegen/boxAgainstJava/notNullAssertions/RightElvisOperand.java

@@ -1,5 +0,0 @@
-class RightElvisOperand {
-    static String foo() {
-        return null;
-    }
-}

compiler/testData/codegen/boxAgainstJava/notNullAssertions/delegation.kt

@@ -1,3 +1,15 @@
+// FILE: Delegation.java
+
+public class Delegation {
+    public static class ReturnNull {
+        public String foo() {
+            return null;
+        }
+    }
+}
+
+// FILE: 1.kt
+
 interface Tr {
     fun foo(): String
 }
@@ -18,4 +30,4 @@ fun box(): String {
         println(e.message)
         return "OK"
     }
-}
+}

compiler/testData/codegen/boxAgainstJava/notNullAssertions/rightElvisOperand.kt

@@ -1,3 +1,13 @@
+// FILE: RightElvisOperand.java
+
+class RightElvisOperand {
+    static String foo() {
+        return null;
+    }
+}
+
+// FILE: 1.kt
+
 fun baz(): String? = null
 
 fun bar(): String = baz() ?: RightElvisOperand.foo()