Move or delete spoon testing package from src/main

Currently, we have assertions/testutils in `src/main` instead of `src/test` see https://github.com/INRIA/spoon/tree/master/src/main/java/spoon/testing. Given the java 17 release will be an already breaking major release, I propose to either remove or move these classes. A good replacement is either the default junit methods or we create our on SpoonAssertions extending AssertJ , see https://joel-costigliola.github.io/assertj/assertj-core-custom-assertions.html.

Inspect the classes under src/main/java/spoon/testing and their usage from the test sources. Decide whether they should be removed, moved to src/test, or replaced with SpoonAssertions based on the existing proposal; done means the testing utilities are no longer incorrectly shipped from src/main and the test suite still passes.

Written by the indexing model from the issue text.