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