Refactor integration test

[danschultzer]

Stacked on #448.

Updates the dependencies being tested with, uses Phoenix formatter rules, and refactors the integration test with the following changes:

1. No need to set up two ports at the same time since we only test a single endpoint for each test
2. Use test tags for opts/config
3. Detach only the telemetry handlers we know are attached
4. DRY setup and request handling