[bitnami/keycloak] Revisit Goss/Cypress tests

[FraPazGal]

Description of the change

After reviewing the existing tests and the general testing strategy we have been following until now, we saw there was a need to revisit them. The main objective that our chart testing strategy should aim for is asserting that the Chart deliverable works. As such, the focus should not be on the application itself or the container, which are regarded as trustful components (that is, they should have been tested at a previous stage).

As charts usually pack a number of different components, it is essential to test that their integrations and the application itself work as expected.

Consequently, we have updated the tests so they are related to the chart deliverable and left only a couple of basic tests to verify that the application functions correctly. Besides that, an effort has been made to reduce future tests' flakiness improving some existing selectors and checks.

Tested in https://github.com/FraPazGal/charts/pull/73

The VIB Verify executions failed due to the trivy action (now removed from bitnami/charts no longer present in bitnami/charts

Checklist

• [x] Chart version bumped in Chart.yaml according to semver. This is not necessary when the changes only affect README.md files.
• [x] Variables are documented in the values.yaml and added to the README.md using readme-generator-for-helm
• [x] Title of the pull request follows this pattern [bitnami/<name_of_the_chart>] Descriptive title
• [x] All commits signed off and in agreement of Developer Certificate of Origin (DCO)