Whitelist test for underscore builtins

[blaggacao]

I think there should be at least in test a whitelist test for underscore builtins, to make sure we detect early if new nix versions add a new underscore builtin that we'd have to switch off for its character:

https://github.com/ekala-project/modules/blob/master/test%2Fbld%2Fmod.nix