Introducing find bugs maven plugin on tests too

This is a follow-up to Pull Request #176

This PR includes various fixed to issues reported by FindBugs on "test" source-trees.

Most issues have been fixed.
Some other issues are just ignored with @edu.umd.cs.findbugs.annotations.SuppressWarnings annotations, when the intention was not clear whether or not this was done on purpose.

At the end, FindBugs plugin is configured to analyse the "test" source tree too.