Added an example of testing the rendered template to guidelines

What does this MR do?

Currently, Frontend testing guideline about "not testing a library" includes only an example of 'bad' implementation. This might lead to confusion as people tend to read code snippets without checking the context sometimes. I've tried to add an example of the desired implementation and mark examples as bad and good respectively.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides