refact(tests): improves handling of cleanup func

[supitsdu]

Handles cleanup using t.Cleanup() instead of manually calling defer after using replaceStdin.

Related review: #19 discussion

[ccoVeille]

That's the main reason why t.Cleanup was added: avoid the callbaxk that you need to return and defer everywhere you call the method

That's why t.Cleanup supersedes defer in tests.

So when I see a defer in tests I ask to use t.Cleanup

Also, the defer and t.Cleanup registration order differs. It helps in a few cases

Here are a few articles

https://brandur.org/fragments/go-prefer-t-cleanup-with-parallel-subtests#:~:text=The%20fix%20is%20t.,with%20pauses%20caused%20by%20t.
https://stackoverflow.com/questions/61609085/what-is-useful-for-t-cleanup