refactor: move tests resources

As pointed out in #2136 the tests directory is a bit messy, especially when you're not familiar with Spoon contribution. I would suggest to refactor it at least to have properly defined rules for the test resources.

We currently have test resources, in at least 3 different places:
1. `src/test/resources` the standard directory
2. `src/test/java/X/testclasses` in a subpackage, along with their test suite
3. `src/test/java/X` in the same package of the test suite

So I suggest first to move the test resources from case 3 to put - at least - the resources in a subpackage `testclasses`.
I don't think it's desirable to move all test resources in place 1, as I think we want to check that we can compile them.

WDYT?