Follow-up: Add a feature spec for the dashboards visualization designer

The following discussion from !110654 (merged) should be addressed:

• @mrincon started a discussion: (+3 comments)

  Thanks for this @rob.hunt & @mwoolf, after seeing this, do you think there is a way to ensure the dashboards are working with a feature spec?

  I am not sure which was the problem specifically (I assume the change in ee/app/assets/javascripts/product_analytics/dashboards/constants.js) but since this was not detected by out jest tests, I would hope to have an additional check. "TrackedEvents.count"

Implementation plan

frontend - 2️⃣

1. Add a feature spec validating that the visualization designer (internal link) is shown and basic functions work as expected:
   • Click on a measurement
   • Then click on a dimension
   • Then confirm that the three panel headers are shown: Data, Panel, Code.
   • Then confirm that the data panel is shown with and it includes the mocked cube response data.

Additional notes:

• We can follow-up with more specs for the different visualizations.
• Look at ee/spec/features/projects/product_analytics/dashboards_spec.rb for examples on how to do this.
• This is not a full end-to-end spec. Cube responses should be mocked.