test(test_config.py): add tests for circular transclusion failure case

[RR5555]

Issue

Circular transclusions lead to:

E           RecursionError: maximum recursion depth exceeded while calling a Python object

/home/gitpod/.local/share/uv/python/cpython-3.9.23-linux-x86_64-gnu/lib/python3.9/pathlib.py:646: RecursionError

It might be nice to catch it as soon as it happens, and provide a more comprehensive error message including the names of the files that collide.

---

This PR

This PR introduces tests for circular transclusions that shows that these are not really dealt with in a user-friendly manner in addition to taking some unnecessary time.
The tests are currently skipped to not interfere with normal condition testing, and commented with a TODO line similar to the last one in the previous section Issue.

NOTE: The test with demo_transclude_circular_bis will fail because of the circular recursion.
The test with demo_transclude_circular will:

• currently fail with a yaml.composer.ComposerError: expected a single document in the stream
• if https://github.com/copier-org/copier/pull/2251 is accepted, also fail because of the circular recursion.
  But it might be a better test as it needs to go to the deepest transclusion to get the _excludes.

[sisp]

Thanks for catching this bad error management and providing test cases, @RR5555! 🙇

We might even be able to add cycle detection with just some minor change. The YAML loader with support for the !include constructor is built inside the load_template_config() function. As the !include handler is a closure in this function, we could record the included YAML files in a set created in this function,, and when including another YAML file which has been included before, we could raise a proper error like CircularConfigFileIncludeError (declared in copier/errors.py). WDYT?