Don't detect ocaml-ci specifically, improve CI detection

[MisterDA]

In light of recent discussions (see #394 #395), I think that simply using the CI environment variables to enable the show_errors behavior is sufficient, if there's nothing more to do to integrate the CI with Alcotest. Turns out, we were already doing that with the `Unkown variant.

AppVeyor uses CI=True (capitalized), support that too. Couldn't find if Jenkins defines a specific variable.