Improve unit testing for seed_data()

[amhanson9]

Split into more functions and create tests for each function. Include tests for the CSV contents and not just the dataframe. Wait to do this until switch to CSV input for warc_download.py, since it will require a rework of this function.

[amhanson9]

Instead of having one unit test per department, do one unit test per data variation. All the variations appear to be covered by the date ranges chosen, but it isn't explicit what is being tested by each. Variations:

• Archive-It collection once
• Archive-It collection more than once
• Job once
• Job more than once
• Seed with one WARC
• Seed with multiple WARCs