[CI] Unit tests only test the most recent chart version

Summary

Our unit tests currently only test the latest version available from the CHART_VERSIONS file. We should ensure that CI runs unit tests against all available chart versions.

Note that we should probably only start with unit tests and exclude functional tests for now. Running 3 parallel functional tests for each MR pipeline will likely overwhelm our CI clusters.

Acceptance criteria

• Tests check the environment for CHART_VERSION, and if not set, defaults to the latest version from CHART_VERSIONS file
• CI runs unit test jobs for each available version