[11.x] Update tests to ensure mail Message implements the fluent interface pattern

[seriquynh]

Because Illuminate\Mail\Message implements the fluent interface pattern. Each of these methods: from, sender, returnPath, to, cc, bcc, replyTo, subject and priority must change and return the instance itself. But current tests can be passed if a method return a clone/new static Message instance.

Let's take a look at from method.

<?php

public function from($address, $name = null)
{
    $msg = clone $this; // Clone the called object.

    is_array($address)
        ? $msg->message->from(...$address)
        : $msg->message->from(new Address($address, (string) $name));

    return $msg; // Return the cloned one.
}

public function testFromMethod()
{
    # These assertions are still passed.
    $this->assertInstanceOf(Message::class, $message = $this->message->from('[email protected]', 'Foo'));
    $this->assertEquals(new Address('[email protected]', 'Foo'), $message->getSymfonyMessage()->getFrom()[0]);

    # But these won't because we check the message instance itself.
    $this->assertSame($this->message, $this->message->from('[email protected]', 'Foo'));
        $this->assertEquals(new Address('[email protected]', 'Foo'), $this->message->getSymfonyMessage()->getFrom()[0]);
}

I think if a class implements the fluent interface pattern, assertSame is the required assertion when writing tests.