[Server] Testing order matters that DB.test.js is the first to be run

[devYaoYH]

Potential test ordering fix for Jest:
jestjs/jest#6194 (comment)

Currently we are doing:

1. Clear DB in yarn pretest script
2. Populate DB in jest globalSetup script
3. Run tests

OK:
DB.test.js -> routes.test.js
Problem:
routes.test.js -> DB.test.js as we are adding an item to Orders Collection in one of our tests in routes.test.js. This results in one test in DB.test.js to fail as we do not expect this added item.

Solutions:

1. Avoid side effects by dropping this document after we assert it has been successfully added
2. Refactor code such that we clear DB and re-populate in jest beforeAll for each test suite (but then DB.test.js isn't really that useful anymore?)

For now, I've taken the earlier approach

Originally posted by @devYaoYH in #104

[devYaoYH]

SGTM!
2# seems like a better approach in long run! Perhaps just copy over the DB from DB.test.js into a new one in beforeAll and then delete it in AfterAll. The items stay neat then.

Originally posted by @BhavyaLight in #104