tests/config/yaml: use pytest_collect_file() hook for YAML files

tests/config/yaml: use pytest_collect_file() hook for YAML files

This patch changes `tests/config/yaml/conftest.py` so that it implements
a pytest_collect_file() Pytest hook to create Pytest file and item
objects for each failing barectf YAML configuration file found.

This makes is possible to remove all `test_fail_*.py` files and avoid
duplication and mismatches between existing YAML files and available
test functions.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>