Proposal: Implement methods for RSpec testing of ERB

Summary

We do not have a good way to validate the ERB code rendering. We should be able to find provide a means for testing this via RSpec, through some libraries. We should investigate / implement methods to be able to mock and test in-container rendered template via RSpec.

Current behavior

Can not test ERB template functionality, as rendered by these charts.

Expected behavior

Able to test ERB template functionality, as rendered by these charts.

Acceptance

• Provide means to mock content / environment
• Provide means to call ERB per implemented / documented patterns
• Implemented as a helper library to be consumed as part of the RSpec patterns within this chart
• Example implementation MR to consume this and/or documentation added to styleguides for future use
  • RSpec: add helper for ERB and gomplate (!3289 - merged)
  • Document ERB and gomplate test helpers (#4898)