Add check to the wrapper's meta.tests

[multivac61]

Is your feature request related to a problem? Please describe.

@phaer created an issue in blueprint that explains the problem quite well.

I love simply adding a formatter nix to invoke treefmt-nix by just adding a treefmt-nix.mkWrapper call as e.g. in nixos-facter. But I'd like to use the same treefmt-nix configuration as a flake check. As afaik there's no generic interface to tell a given formatter to enter "check mode", I believe we can't add such a feature to blueprint automatically.

Describe the solution you'd like

treefmt-nix add the check to the wrapper's meta.tests.

Describe alternatives you've considered

Being sad about it and doing nothing.

Additional context

numtide/blueprint#31