[token-server] Improve organization of test files

Summary:
The whole "recreate src dir structure in test dir" is actually pretty annoying.

Keep the test files next to the files they are testing. Keep the test dir around for test-related mocks and vectors. Update tsconfig.json so that we stop getting lint errors on the tests.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17062