Add options for --omit-tests and --omit-benchmarks

[locallycompact]

This address this https://github.com/NixOS/cabal2nix/issues/531

Kind of, it's independent of the dontCheck attribute. If --no-check isn't on but --omit-tests is, then the build will fail with a missing dependency. I'm not sure how to split this out properly but it gets rid of the unnecessary evaluation inputs in the case where the tests aren't used.