Meta: Please, add tests to verify string context behavior

  * [ ] `builtins.nixPath`
  * [ ] `builtins.fetchurl`
  * [ ] `builtins.findFile`
  * [ ] `builtins.fromJSON`
  * [ ] `builtins.exec`
  * [ ] `builtins.unsafeGetAttrPos`
  * [ ] `builtins.mapAttrs`
  * [ ] `builtins.readFile`
  * [ ] `builtins.readDir`
  
  For examples, see: https://github.com/haskell-nix/hnix/pull/474/files & 13fc5bc.