Add testing

[flxw]

Tooling is available to perform testing of the Helm charts.
However, not every chart undergoes the same amount of testing uniformly or provides the same level of guarantee.
Having testing in place is a prerequisite for us to confidently refactor and evolve the charts.

Acceptance criteria:

• Covered all sub-charts with the same level of testing
• Added end-to-end testing that spins up a whole Sigstore cluster and signs/verifies an image

Determine whether the existing tooling is sufficient to provide the desired test coverage and align each of the charts so that they undergo the same level of rigor.