getmail: fix configuration mailboxes generation (#1719)

The mailboxes must be a tuple of string or the string "ALL".
The generated value was broken if the mailboxes configuration was a list
of only one string (but not "ALL"): the generated expression ( "str" )
was not a tuple but a string.
Now, we always generate a tuple (by adding a comma, even with a list of
size one). Getmail works with the special value "ALL" whether it is a
in tuple or not, so this case is not specifically handled.

modules/programs/getmail.nix

@@ -11,8 +11,7 @@ let
     with account;
     let
       passCmd = concatMapStringsSep ", " (x: "'${x}'") passwordCommand;
-      renderedMailboxes =
-        concatMapStringsSep ", " (x: "'${x}'") getmail.mailboxes;
+      renderedMailboxes = concatMapStrings (x: "'${x}', ") getmail.mailboxes;
       retrieverType = if imap.tls.enable then
         "SimpleIMAPSSLRetriever"
       else

tests/modules/programs/getmail/getmail-expected.conf

@@ -5,7 +5,7 @@ server = imap.example.com
 port = 993
 username = home.manager
 password_command = ('password-command')
-mailboxes = ( 'INBOX', 'Sent', 'Work' )
+mailboxes = ( 'INBOX', 'Sent', 'Work',  )
 
 [destination]
 type = MDA_external