[8.x] Make mailable assertions fluent

[macbookandrew]

This adds a bit of nice DX so mailable assertions are fluent similar to HTTP assertions.

Before:

$mail = new WelcomeEmail($user);

$mail->assertSeeInHtml('Welcome, '.$user->name.'!');
$mail->assertSeeInText('Welcome, '.$user->name.'!');

After:

$mail = new WelcomeEmail($user);

$mail
    ->assertSeeInHtml('Welcome, '.$user->name.'!')
    ->assertSeeInText('Welcome, '.$user->name.'!');

// Alternative

(new WelcomeEmail($user))
    ->assertSeeInHtml('Welcome, '.$user->name.'!')
    ->assertSeeInText('Welcome, '.$user->name.'!');