governance: performing checkstyle on src/test

Since this summer @zielint0 is using a lot of energy for improving Spoon, mostly by fixing checkstyle issues in our test-base.

We discussed a lot at the beginning about the benefit for doing that in Spoon, since the checkstyle is not enforced in the test: I personally was mostly worrying that this energy would be lost, because the next PR would bring new checkstyle issues to fix and here becomes an endless circle.

Now we can see that it's indeed what happens: it seems that the number of PR for fixing checkstyle is not decreasing, and it's almost the contrary.

So, in order to save the energy of @zielint0 and ours when reviewing his PR, I propose that we start to enforce the checkstyle in the test: we do not have to use the same checkstyle as in Spoon though, it could be a lighter one, or at least we might be able to configure it to skip `src/test/**/testclasses`.

WDYT?