Check: build passthru.tests when present

[Ekleog]

This is my first attempt at haskell in a quite long time, and I haven't been able to nix-build the file produced by cabal2nix even before my changes (maybe due to my being on 19.03?), so it's quite possible that this does not even parse.

Other than that, I've tried to keep things consistent, and hopefully this should work!

[Ekleog]

(Replacing <nixpkgs> with channel:nixpkgs-unstable in the shell.nix allowed me to test locally)

[Ekleog]

(all stupid errors fixed locally, travis should pass now)

[ryantm]

Thanks!

[ryantm]

NixOS/nixpkgs#63216
@Ekleog
Looks like something is going wrong because it is creating a warning for packages without tests.

[ryantm]

pushing /nix/store/s7j02mg1fg6nxiwy0yih5i1fscvq3sih-brial-1.2.5
All done.
mktemp -d
/tmp/tmp.VanUS1zMPa
nix-build --option sandbox true --option restrict-eval true --arg config '{ allowBroken = true; allowUnfree = true; allowAliases = false; }' -E '{ config }: (
import ./. { inherit config; }).brial.tests or {}'
error: access to path '/tmp/tmp.VanUS1zMPa' is forbidden in restricted mode

[Ekleog]

@ryantm I've submitted a tentative fix in #119 :)