Split test fixtures

[caarmen]

• Move test data to a "testdata" package.
• Move the mock datetime utility to a mocks package.
• Move pytest fixtures out of conftest.py and into separate plugin packages.

[github-actions]

File	Coverage
All files	98%	✅
logger.py	91%	✅
main.py	97%	✅
settings.py	99%	✅
data/database/connection.py	92%	✅
domain/localrepository/localwithingsrepository.py	94%	✅
domain/usecases/slack/usecase_post_daily_activity.py	94%	✅
domain/usecases/slack/usecase_post_sleep.py	97%	✅
remoteservices/api/withings/subscribeapi.py	83%	✅
remoteservices/api/withings/weightapi.py	93%	✅
remoteservices/repositories/webapifitbitrepository.py	97%	✅
routers/dependencies.py	85%	✅
tasks/fitbitpoll.py	96%	✅
tasks/post_daily_activities_task.py	92%	✅

Minimum allowed coverage is 80%

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