fix: add requirements to run tests

[theletterjeff]

Integration tests have not been running on contributors' machines due to missing dependencies from the requirements.txt file. This PR adds the four missing test dependencies.

[AuHau]

Check out the Contributor guideline 😉

https://github.com/AuHau/toggl-cli/blob/master/CONTRIBUTING.md

There it is stated that you need to install different target for the test dependencies.

But that said it could be improved with adding that you can also install it with pip install -r test-requirements.txt

[AuHau]

Thanks for the PR, but the distinction between testing dependencies and runtime dependencies is there for a reason, so I won't merge this.