Add GitHub action to check if tests pass

[aruZeta]

• Create a nimble task for running tests (don't like the default one)
• Create the action
• Doc generation workflow runs only when the tests have passed or when there was a push to main

[aruZeta]

Also would be nice to make the doc generation workflow run only when the tests have passed or when there was a push to main.

Added as a task.

[aruZeta]

Seems like it works, but it's not actually listed in the commit's run workflows:

It would be nice that, when I create the README, I add badges for both workflows (even pages too).

[aruZeta]

Also I actually only pushed the inputs of the flake to my cachix cache, so nim had to be built all over again and the action took around 8-10 minutes.

I have already pushed the actual outputs and now the actions seem to take around 2 mins!

So, since all the tasks have been completing, I'm closing this issue.