Tests: Add unique messages to assertions for attachment filenames in `wp_mail()`.

This makes the assertions more helpful, as per the [https://make.wordpress.org/core/handbook/testing/automated-testing/writing-phpunit-tests/#using-assertions Writing PHP Tests] guidelines:
> All PHPUnit assertions, as well as all WordPress custom assertions, allow for a `$message` parameter to be passed. This message will be displayed when the assertion fails and can help immensely when debugging a test. This parameter should always be used if more than one assertion is used in a test method.

Follow-up to [55030], [55032].

Props mukesh27, costdev.
See #28407.
Built from https://develop.svn.wordpress.org/trunk@55087


git-svn-id: http://core.svn.wordpress.org/trunk@54620 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.2-alpha-55086';
+$wp_version = '6.2-alpha-55087';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.