Feature Request: Newsletter - Add `[Test]` to the subject line of test emails

[rosepajaroja]

Impacted plugin

Jetpack

What

Add [Test] to the subject line of test emails sent when users are previewing or testing newsletters.

How

When a user sends a test email from the newsletter editor, prepend [Test] to the subject line programmatically before dispatch. For example, if the original subject is March Travels, the test email subject would appear as [Test] March Travels.
This change should apply only to test emails and not affect the subject of the actual newsletter when it's sent to subscribers.

This makes it easier for users to distinguish test emails from live newsletters in their inbox. It helps prevent confusion, reduces the risk of accidentally forwarding or acting on a test email as if it were live, and aligns with common email testing practices.

[github-actions]

OpenAI suggested the following labels for this issue:

• [Feature Group] Content Management: The issue relates to managing the content of newsletters within the Jetpack plugin.
• [Feature] Jetpack: The Jetpack plugin is specifically mentioned as being impacted by the feature request.
• [Feature] Subscriptions: The request is about sending test emails for newsletters, which relates to the subscription and email functionality.