[alias-server] Better organization for db unit tests

Summary:
T3060

Depends on D13705

Move before and after hooks inside the describe statement to make sure they only impact this test suite.
Add beforeEach and afterEach hooks to initialize and wipe the db in between each test

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13706