[OPS] Update Github offline_tests workflow to run SlackApp tests

[oshoma]

PROBLEM
In .github/workflows/offline_tests/yml we run the tests found in src/sherpa_ai. This is good. However, we do not run the Slackapp tests, which are found in src/apps/slackapp/tests.

SOLUTION
Update offline_tests.yml as follows:

1. Change working directory to src/apps/slackapp/
2. Install poetry and dependencies in the slackapp directory, similar to what we do already for sherpa_ai
3. Run slackapp tests: poetry run pytest --cov=slackapp tests

Or take a dependency on #343 and instead of steps 2 and 3, just run make test. That would be a nice way of DRYing up the offline tests action.

OUTCOME

• Increased test coverage
• Some insurance that changes do not break the Slack app

[oshoma]

Closing this issue, since the Slack App feature of Sherpa is deprecated.