Unit tests (even server unit tests) should not make network calls

[krivard]

The following three tests failed when one of our volunteers was trying to run unit tests without having set up the integration test prerequisites. This shouldn't have been a problem, since the point of unit tests is to check the smallest units of functionality, mocking out more complex functions (like network connections).

• server/test_pandas.py
• server/endpoints/test_covidcast.py
• server/endpoints/test_nidss_flu.py

It's not immediately obvious why these tests were making network calls -- I suspect that something in _common attempts to connect to the database on import.

We should do one of the following:

• (ideal) figure out how to properly mock out network calls for flask tests
• (or if that is not possible) move these tests to the integrations section