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.
Before the XDG variables would be set from the user's environment, if
available. This would break some tests.
With this change the tests should be fully deterministic.
Fixes#1190
