Add more unit tests (#390)

* Add unit tests for server settings

* Add more tests

* Update version of helm-unittest

* Add tests for prefect-worker chart

* Move helm chart test to correct directory

* prometheus-prefect-exporter: fix template name in prometheusrule.yaml

* Add tests for prometheus-prefect-exporter chart

* helm_unittest script: test all charts

Now that we have tests for all of them.