Create Preparation model unit tests

Test if all fields are of proper type, and that only the expected fields
exist.
Make preparation model tests actually work
Rewrite preparation model tests based on Vendor model tests

Move into tests folder and use a dictionary to track test
Change unicode test string to ascii
Create Preparation model
Test the correct model (duh)
Create Preparation model unit tests

Test if all fields are of proper type, and that only the expected fields
exist.
Make preparation model tests actually work
Rewrite preparation model tests based on Vendor model tests

Move into tests folder and use a dictionary to track test
Change unicode test string to ascii
Create Preparation model
Test the correct model (duh)
Merge branch 'feature/16575' of https://github.com/osu-cass/whats-fresh-api into feature/16575

Conflicts:
	project_fish/whats_fresh/models.py
	project_fish/whats_fresh/tests/__init__.py