Currently the following, will produce `some_opt=false` in the
rendered config:
```nix
programs.mangohud.settings = {
some_opt = false;
};
```
With the intention being to disable the option, this would be
incorrect, as per the following stated at:
<0575c8eb1f/data/MangoHud.conf (L3C5-L4C1)>
> Use some_parameter=0 to disable a parameter (only works with on/off
> parameters)
As such, I changed the rendering to follow this.
This will be output instead: `some_opt=0`
This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
