tests break if some helm template fails

[hervenicol]

Related to: #1432

We noticed that tests were not failing when a helm template was failing to render, which made spotting the issue harder.
With this change the tests would have failed right when the YAML parse error happens, making the root cause of the failure obvious.

Checklist

• Update CHANGELOG.md
• Add Unit tests
• Follow Alert structure
• Consider creating a dashboard (guidelines) (if it does not exist already) to help oncallers monitor the status of the issue.
• Request review from oncall area, as well as team (e.g: oncall-kaas-cloud GitHub group).