Align the `PopupMenu` under its child.

Before:

![image](https://user-images.githubusercontent.com/32538273/234468262-beddb257-5f5a-422f-a678-c26efc52dc07.png)

After:

![image](https://user-images.githubusercontent.com/32538273/234468233-3ea4d741-b04c-4012-bdd7-a36ecedc029c.png)

*List which issues are fixed by this PR. You must list at least one issue.*

Fixes https://github.com/flutter/flutter/issues/125474

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*