Improve tests in Spoon

I just had a report from @oscarlvp who is working on software testing that we have a lot of methods not properly tested in Spoon. For a lot of them we have a coverage, but removing the body of the method does not change at all the test results. For other methods, deciding an arbitrary return value (like return true) does not change either the test results.

So we should improve our test basis. The complete list of badly tested methods (and why) is available there: https://github.com/STAMP-project/descartes-experiments/blob/master/spoon/index.md