Add tests

[davorg]

Lots of tests needed.

[JJ]

There are not many tests you can do without qmail installed... And that would need to be added to the configuration, too. Short of showing the CL options, will see what can be done.

[choroba]

Well, you can mock qmail to some extent, but in some places a rewrite of the code might be needed to make it possible (e.g. when opening files ${queue}mess/$msg_id it would be handy to have a way of injecting an existing filehandle instead). An example can be seen in my fork but it shows there are more things to change in the code (e.g. info messages shouldn't be warnings but print STDERR so we don't have to account for them all the time when testing warnings).