Update tests.

Author: Alexander Regueiro

src/test/ui/error-codes/E0225.rs

@@ -1,4 +1,10 @@
+#![feature(trait_alias)]
+
+trait Foo = std::io::Read + std::io::Write;
+
 fn main() {
     let _: Box<std::io::Read + std::io::Write>;
     //~^ ERROR only auto traits can be used as additional traits in a trait object [E0225]
+    let _: Box<Foo>;
+    //~^ ERROR only auto traits can be used as additional traits in a trait object [E0225]
 }

src/test/ui/error-codes/E0225.stderr

@@ -1,9 +1,18 @@
 error[E0225]: only auto traits can be used as additional traits in a trait object
-  --> $DIR/E0225.rs:2:32
+  --> $DIR/E0225.rs:6:32
    |
 LL |     let _: Box<std::io::Read + std::io::Write>;
    |                                ^^^^^^^^^^^^^^ non-auto additional trait
 
-error: aborting due to previous error
+error[E0225]: only auto traits can be used as additional traits in a trait object
+  --> $DIR/E0225.rs:8:16
+   |
+LL | trait Foo = std::io::Read + std::io::Write;
+   |                             -------------- non-auto additional trait
+...
+LL |     let _: Box<Foo>;
+   |                ^^^ expanded from this alias
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0225`.