compile_fail shouldn't run successfully

[mauvealerts]

This follows up on @mgeisler's comment on #3. While working on this, I realized there's some wrinkles with "weird" test configs that this doesn't address. Validation seems desirable, but there are some details I'm unsure of. I filed #7 for the moment.

[mgeisler]

Thanks, without knowing the code, this looks good 😄