Enable warnings-as-errors in mix test

[kianmeng]

This supposes to catch those deprecation warnings in Elixir 1.15,16,17 and Erlang/OTP 25,26,27.

Follow-up discussion in #152 (comment)

[RobertDober]

I'll have merged this anyway because I see your idea (I guess), suspecting that the compiler itself might be more linient with warnings in tests.

Stupid me, now I get it, .exs files are not compiled in the mix compilestep. 👏

Thanx and take care

[RobertDober]

@kianmeng fyi a little bit more was necessary to get the compilation and test step run for the older versions again

Here is what was missing 5228fce

But thanx for putting me on track

[kianmeng]

🥳 🥳 🥳 🥳 🥳