Add test to prevent further Service Output interpolation regressions

[atc0005]

Overview

Apply a series of changes to help prevent further regressions:

• move Service Output handling logic to helper function
  • consolidate logic in one place similar to how the other output sections (e.g., Errors, Long Service Output) are already handled to make this logic easier to test
• add test case
  • intentionally restore regression by using fmt.Fprintf function in helper code
  • fix regression by using fmt.Fprint as intended, confirm expected behavior

References

• GH-58
• GH-139