Run helm dependency update prior to helm tests

Since sylva-projects/sylva-elements/helm-charts/sylva-capi-cluster!318 (merged), we need to run helm dependency update to fetch sylva-library dependency chart.

Without this change, as error as in https://gitlab.com/sylva-projects/sylva-elements/helm-charts/sylva-capi-cluster/-/jobs/7641743281 would be seen.

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Successfully tested in https://gitlab.com/sylva-projects/sylva-elements/helm-charts/sylva-capi-cluster/-/pipelines/1423227894.