Migrate boxWithJava tests to multi-file framework

compiler/testData/codegen/boxWithJava/notNullAssertions/extensionReceiverParameter.kt

@@ -1,3 +1,5 @@
+// FILE: Test.java
+
 public class Test {
     public static String invokeFoo() {
         try {
@@ -15,3 +17,13 @@ public class Test {
         return "Fail: assertion must have been fired";
     }
 }
+
+// FILE: extension.kt
+
+fun Any.foo() { }
+
+val Any.bar: String get() = ""
+
+fun box(): String {
+    return Test.invokeFoo()
+}

compiler/testData/codegen/boxWithJava/notNullAssertions/extensionReceiverParameter/extension.kt

@@ -1,7 +0,0 @@
-fun Any.foo() { }
-
-val Any.bar: String get() = ""
-
-fun box(): String {
-    return Test.invokeFoo()
-}