Simple cipher remove test: do not check what happens with invalid input

[jiegillet]

Fix #851.

I'm not pushing this particular fix, dropping "only lowercase a-z are translated, rest are passed through" is another valid resolution, but I made the change on my work laptop, and I probably won't have it later. Feel free to pick your preferred fix @angelikatyborska :)

PS: sorry for the messy commits, I have to rush a bit.

[github-actions]

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[jiegillet]

Did I close this? I didn't mean to... Oh, I think I deleted the branch by mistake, maybe that's what happened...