test(package): Show existing alt reg --list behavior

Author: epage

tests/testsuite/package.rs

@@ -6791,6 +6791,15 @@ fn registry_not_in_publish_list() {
 [ERROR] `foo` cannot be packaged.
 The registry `alternative` is not listed in the `package.publish` value in Cargo.toml.
 
+"#]])
+        .run();
+
+    p.cargo("package --registry alternative --list")
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[ERROR] `foo` cannot be packaged.
+The registry `alternative` is not listed in the `package.publish` value in Cargo.toml.
+
 "#]])
         .run();
 }