modules/output: check warnings+assertions on build.package

Add a `build.packageUnchecked` option for use instead of the old `check`
evalNixvim argument.

lib/tests.nix

@@ -66,9 +66,6 @@ let
         extraSpecialArgs = {
           defaultPkgs = pkgs;
         } // extraSpecialArgs;
-        # Don't check assertions/warnings while evaluating nixvim config
-        # We'll let the test derivation handle that
-        check = false;
       };
     in
     result.config.build.test;