test: fix relative paths for downstream ci

[VeraChristina]

Description

Adds a fixture to the integration test set-up that navigates to the repo's root dir at the beginning of each test.
This is needed because we use config files in anemoi-core/training//src/anemoi/training/config/ with relative paths for hydra, as well as config files from the integration test dir in anemoi-core.

This fixes the fast integration tests failing on the downstream-ci where tests are not run from the root dir of anemoi-core.

Error message from downstream-ci on anemoi-utils:
=========================== short test summary info ============================

ERROR tests/integration/test_training_cycle.py::test_config_validation_architecture_configs[architecture_config0] - FileNotFoundError: [Errno 2] No such file or directory: '/opt/actions-runner/work/_work/anemoi-utils/anemoi-utils/training/training/tests/integration/config/testing_modifications.yaml'

ERROR tests/integration/test_training_cycle.py::test_config_validation_architecture_configs[architecture_config1] - FileNotFoundError: [Errno 2] No such file or directory: '/opt/actions-runner/work/_work/anemoi-utils/anemoi-utils/training/training/tests/integration/config/testing_modifications.yaml'

============ 92 passed, 3 skipped, 104 warnings, 2 errors in 10.81s ============

Error: Process completed with exit code 1.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Code Compatibility

• I have performed a self-review of my code

Code Performance and Testing

• I have run the integration tests from different directories to check if my fix is effective

Documentation

• My code follows the style guidelines of this project
• I have updated the documentation and docstrings to reflect the changes
• I have added comments to my code, particularly in hard-to-understand areas