Add unit tests and CI workflow

[QSparks]

Add CI workflow and unit tests for Translation, helpers, and app endpoints.
Resolves #6

[rod-glover]

Wow, very nice work! This repo is much the better for these tests.

One question: What advantages do you think we get by using test classes rather than flat collections of functions?

[rod-glover]

Test fixtures are usually stored in conftest.py files. Any reason not to do that here?

[QSparks]

That's a good point. I left them in this file to keep things readable, since they’re only used here.
If our fixtures end up being reused elsewhere, we can move them to conftest.py.

[QSparks]

What advantages do you think we get by using test classes rather than flat collections of functions?

Using test classes gives additional context in our logs. While not strictly necessary here, it becomes useful if we add tests for other classes in the same file, as it scopes test failures more clearly.

Context is also factored out into the class, shortening test names.
Example:
FAILED tests/test_routes.py::TestAppEndpoints::test_dynamic_retries_on_failure