Add tests for RecipientHandler

Looks good Adam, the coverage seems fine.

It took me a little work to figure out how exactly the logic flowed, the following are just some unimportant notes on what might have helped me get it quicker.

1. +++ b/src/Tests/SimplenewsRecipientHandlerTest.php
   @@ -0,0 +1,146 @@
   +    // Mark users 4&5 as logged in.
   ...
   +    $this->assertText(t('Send newsletter issue to 3 subscribers.'));
   

   The number of users found is not the number the comment leads us to expect.

2. +++ b/src/Tests/SimplenewsRecipientHandlerTest.php
   @@ -0,0 +1,146 @@
   +    foreach (array_slice($users, -2) as $user) {
   ...
   +    $this->checkRecipients(array_keys(array_slice($users, 0, 3)));
   

   It took me a while to figure out what was going on in these 2 different slices; it seems fine, albeit could be more explicit.

3. +++ b/src/Tests/SimplenewsRecipientHandlerTest.php
   @@ -0,0 +1,146 @@
   +    $this->checkRecipients(array_keys(array_slice($users, 2, 2)));
   

   There's a lot going on in this line.

   It would be easier to understand if you moved the slicing to higher up in the function where you first worked on the users.

   The significance of array_keys here is not obvious.

   Something like:

   $recipients  = array_slice($users, 2, 2);
   ...
   //Fill in the forms etc.
   ...
   $recipientEmails = array_keys($recipients);
   $this->checkRecipients($recipientEmails);
   

makes sense