Add _nixvimTests = false to flake.lib output (#1125)

In my personal configuration, I rely on `nixvim.lib.${system}.helpers` to get access to things like `mkRaw` and `toLuaObject` for use in my extended `lib`.

With the changes made in 6d7e429537, `nixvim.lib.${system}.helpers` can no longer be accessed directly from the `nixvim` input as `_nixvimTests` is not provided as an input attribute, so loading `helpers.nix` fails.

This small change restores what I *think* is the intended behavior of this module

lib/default.nix

@@ -3,11 +3,12 @@
   makeNixvim,
   makeNixvimWithModule,
   pkgs,
+  _nixvimTests ? false,
   ...
 } @ args: {
   # Add all exported modules here
   check = import ../tests/test-derivation.nix {
     inherit makeNixvim makeNixvimWithModule pkgs;
   };
-  helpers = import ./helpers.nix args;
+  helpers = import ./helpers.nix (args // {inherit _nixvimTests;});
 }