use epidata client in covidcast integration tests

[dfarrow0]

the covidcast integration tests should use the epidata client now that it supports the covidcast endpoints

[krivard]

We want to change

https://github.com/cmu-delphi/delphi-epidata/blob/dev/integrations/server/test_covidcast.py#L27-L35

to use Epidata.covidcast(), the same/similar way

delphi-epidata/integrations/server/test_fluview.py

Line 62 in f949439

response = Epidata.fluview('nat', 202020)

uses Epidata.fluview()

[krivard]

Source for the client: https://github.com/cmu-delphi/delphi-epidata/blob/dev/src/client/delphi_epidata.py