Add tests

[melange396]

This repository is very lacking in terms of tests (automated or otherwise), except for the CI action that makes sure it builds. There are exceedingly basic unit tests in https://github.com/cmu-delphi/www-covidcast/blob/dev/src/data/EpiWeek.spec.ts, https://github.com/cmu-delphi/www-covidcast/blob/dev/src/data/utils.spec.ts, and https://github.com/cmu-delphi/www-covidcast/blob/dev/src/stores/constants.spec.ts. A testing environment was introduced in #604 but disabled and deleted somewhere around or between #757 and #972.

New/more unit tests would be nice, and something like integration tests would be fantastic... But at the very least, we should try to include a few [detailed] smoke tests to mimic the manual poking around that a developer or releaser is supposed to do. They could be automatically run against a PR's associated netlify preview. This can help if/when one of us "forgets" to do those manual checks, or neglects doing them thoroughly.

Associated issues in other repos:

• Add tests www-main#1031
• Add tests www-epivis#73