Test Operator against non-released GitLab Charts

Summary

Currently, the Operator run's the specs against the most-recent released GitLab chart. To catch issues, before the chart version is bumped (usually on a release), we should test against the nightly chart.

This would have cought the bug fixed in !791 (merged) early.

Acceptance criteria

• In addition to versions specified in CHART_VERSIONS, unit tests run against the nightly chart as well.