…for the Mailer (fabpot)

This PR was merged into the 4.4 branch.

Discussion
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[Mailer][Mime] Add PHPUnit constraints and assertions for the Mailer

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | refs #31592, closes #32409, closes #31947, closes #31747, closes #30850
| License       | MIT
| Doc PR        | n/a

This PR introduces PHPUnit constraints and assertions to ease testing emails in functional tests:

```php
<?php

namespace App\Tests;

use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;

class MailerAssertionsTraitsTest extends WebTestCase
{
    public function testSomething()
    {
        $client = static::createClient();
        $client->request('GET', '/');

        $this->assertEmailCount(2);
        $this->assertEmailIsQueued($this->getMailerEvent(0));

        $email = $this->getMailerMessage(0);
        $this->assertEmailHasHeader($email, 'To');
        $this->assertEmailHeaderSame($email, 'To', 'fabien@symfony.com');
        $this->assertEmailTextBodyContains($email, 'Bar');
        $this->assertEmailHtmlBodyContains($email, 'Foo');
        $this->assertEmailAttachementCount($email, 1);
    }
}
```

Commits
-------

23f237b added PHPUnit constraints and assertions for the Mailer