cmd/values: add test coverage

General MR

Summary

I wanted to increase test coverage for the cmd/values function and noticed that a few of our packages could use improvements to make them testable.

This MR:

• Refactors constants.go to be more testable and allow downstream consumers to inject mock clients if desired
• Adds in mockery (as well as a script and make target) to generate testify/mock compatible test fixtures
• Refactors cmd/values.go to be allow more test injections
• Adds additional testing hooks and methods to the FakeFactory and the FakeHelmClient to allow method overriding to force errors
• Increases test coverage of values from ~70% to 90%

Relevant logs/screenshots

Before

Linked Issue

After

Closes #57 (closed) Closes #62 (closed)

Upgrade Notices

N/A