[10.x] Add test for Message ID, References and Custom Headers for Mailables

[alexbowers]

This PR adds some tests for proving that the Message ID, References and Custom Headers are sent as part of the message.

I couldn't find any tests covering this at the moment.

I also wanted to expand the assertX tests available on Mailer, however, from a fairly brief glance, I don't see how to access these headers without the mail actually being sent.

Any guidance on adding the following would be much appreciated:

• assertHasMessageId(string $messageId)
• assertHasReferences(array $references)
• assertHasCustomHeader(string $key, string $value)

[alexbowers]

The purpose of the requested assistance is that I would like to be able to test that I am sending the Mailchimp Subaccount correctly, which is done by adding a header X-MC-Subaccount, but currently I cannot see that exposed in the tests in any way that I can test

[taylorotwell]

@alexbowers can you just call the headers function of your mailable and inspect the returned value?

https://laravel.com/docs/10.x/mail#headers

[alexbowers]

I will give that a try, thanks