release.nix: Make hydra build all unit tests

We have just a few, and these are regular jobs not must-pass. The tests
that were must-pass are left as is.

pkgs/top-level/release.nix

@@ -190,6 +190,8 @@ let
       haskellPackages = packagePlatforms pkgs.haskellPackages;
       idrisPackages = packagePlatforms pkgs.idrisPackages;
 
+      tests = packagePlatforms pkgs.tests;
+
       # Language packages disabled in https://github.com/NixOS/nixpkgs/commit/ccd1029f58a3bb9eca32d81bf3f33cb4be25cc66
 
       #emacsPackages = packagePlatforms pkgs.emacsPackages;