Add runner tests to on-cluster tests

[elliotgunton]

We currently skip all runner tests for the on-cluster tests. I was able to test #1497 on a local cluster by installing the branch through poetry:

poetry add git+https://github.com/argoproj-labs/hera.git#fix-optional-artifact-runner-loading

And using the run make target from the hera example project to export to requirements.txt, docker build will use the git branch build of Hera:

run:
	@poetry export --without-hashes --without-urls > requirements.txt
	@docker build . -t hera-scratch:v1
	poetry run python -m hera_scratch

The logs showed the new optional artifact feature working:

/tmp/hera-inputs/artifacts/my-artifact-path
{"your-value": 3}
{'your-value': 3}
None