core: add new question about the tests to the GitHub template

[neuodev]

Please explain the main changes of your pull request

Whenever a new returned code is added, new unit tests should be written/updated to cover the new case.
I added a checklist item to remind who is opening the PR to add/update the tests.

Checklist

• Have you added any new returned codes to the README file?

[enzoferey]

Hey @AhmedIbrahim336, thanks for the PR !

Just for your information 100% test coverage is enforced via the CI, so we will not miss any tests for sure. But I'm happy to add it to the list 👍🏻

[codecov-commenter]

Codecov Report

Merging #45 (0a83a0f) into main (d4e452b) will not change coverage.
The diff coverage is n/a.

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@@            Coverage Diff            @@
##              main       #45   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          666       666           
  Branches        79        79           
=========================================
  Hits           666       666           

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