Switch unit test from local_daemon to multiprocessing

[jwwwb]

Describe changes

I changed the fresh stack-store fixture to spin up the FastAPI service using the multiprocessing module included with python, instead of the zenml specific local service daemon, which uses the ZEN_GLOBAL directory to store its state. This should allow the unit tests to run in parallel.

Pre-requisites

Please ensure you have done the following:

• I have read the CONTRIBUTING.md document.
• If my change requires a change to docs, I have updated the documentation accordingly.
• If I have added an integration, I have updated the integrations table.
• I have added tests to cover my changes.

Types of changes

• 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 change)
• Other (add details above)

[schustmi]

I think the PR is fine as-is from a code perspective, I do have one alternative suggestion though (which might not be possible or feasible in a reasonable timeframe): As I imagine services will become an integral part of many ZenML integrations/features and will therefore appear in many tests, does it make sense to make this switch from using the daemon code in the service to multiprocessing by patching the LocalService for tests somehow? (Maybe @stefannica has an opinion on this as well)