Add test job to the CI pipeline

[brh28]

Starting with #119, the test suite should now be in a passing state. Moving forward, we want to ensure new releases past this test suite and are being constantly maintained by executing the tests in the Concourse pipeline.

We can likely use Galoy as a reference since we're using the same test runner and make commands.

[brh28]

Good news: Unit tests job running succesfully: https://concourse.flashapp.me/teams/main/pipelines/flash-app/jobs/test-unit/builds/5.4

Bad news: Integration-tests appear to be more involved as the existing Galoy script (test-on-docker-host.sh) is configured to run on a remote GCP instance. Couple options I see:

1. To follow Galoy's approach, we'd need to configure a GCP instance and share credentials
2. To take our own approach, we could spin up an instance in DigitalOcean and change the scripts to run on it.

Either way, there's more complexity in running integration tests than originally expected.

@islandbitcoin let me know how you want to proceed

[brh28]

Standup (1/7) - Look into option 2 above