Create the structure for server unit tests (using Mocha or other)

Currently there are no unit tests for the server-side code.

It be useful to be able to test certain things, especially that badges render properly (eg. visiting `/badge/-test-blue?logo=issue` or similar would return a 200 response code and include the issue icon).

Adding the required file structure and `"test-server"` command in `package.json` would be a very helpful start to get it so `yarn test` or `yarn test-server` would run mocha or a similar testing library.

If you would like to help with this, it is not necessary to implement all of the necessary tests, but even just one or a few would be fine. The code should be able to import existing modules and test their functionality.