Refactor integration tests of server

[krivard]

The integration tests of the server code are arduous to maintain, largely because they violate best practices:

• Avoid copy-pasted code
• Use the least complex example that adequately exercises the code under test

We want to refactor it to address these problems by adding a helper module. This helper module should include (a hierarchy of?) testcase superclasses to support:

• setting Epidata.BASE_URL and secrets
• truncating tables
• loading data
  • should isolate variations so they are easy to read and reason about at the point of use
  • should return a dict in epidata response format using same ordering
  • should feel free to use the associated database.Database as a helper -- these are server tests; database.Database should have been tested elsewhere

The files affected:

• integrations/server/test_covid_hosp.py - has a reasonable example of what loading data might look like
• integrations/server/test_covidcast.py
• integrations/server/test_covidcast_endpoints.py
• integrations/server/test_covidcast_meta.py
• integrations/server/test_covidcast_nowcast.py
• integrations/server/test_fluview.py - setup only
• integrations/server/test_fluview_meta.py - setup only

[melange396]

this example could be a decent start for some of these points: https://github.com/cmu-delphi/delphi-epidata/blob/e0fb58b4cee56e97c29dd81fe54db42a9690e7e4/integrations/acquisition/covidcast/test_db.py

also, collapse multiple definitions of CovidcastRow into one

[krivard]

Fixed in #959