[FEATURE] Improve testing guide to include file naming convention rule

[nathaliellenaa]

Is your feature request related to a problem?

The current testing guide lacks of clear explanation/rule regarding the file naming convention.

What solution would you like?

While a linter for test file naming convention will be added #792, it is important to provide users with a clear guidance on this matter. Therefore, a brief explanation of the file naming convention should be included in the TESTING_GUIDE.md file.

Do you have any additional context?

It's not that easy to follow the peculiar file naming convention in tests, e.g. #787 (review)

[dblock]

I'll take this along with #793