Describe the bug
I discovered recently that test code is being shipped with Darts when installed from pypi. This might be unintentional and I would suggest removing the test code from distributions.

To Reproduce
Steps to reproduce the behavior, preferably code snippet.

from darts.tests.ad.test_aggregators import TestAnomalyDetectionAggregator

Expected behavior
A clear and concise description of what you expected to happen.

System (please complete the following information):

• Python version: 3.13
• darts version: 0.39.0

Additional context
To remove the code, we can exclude the tests folder in MANIFEST.in:

prune darts/tests

Ideally, the tests folder should be separate from the package folder, as suggested by pytest here, unless we want to distribute the test code.