Helps avoid successful build but Sway failing to start.
To meaningfully test this, I had to actually use `pkgs.sway` in the
test rather than the stub, but left all other tests using the stub and
changed them to skipping the test.
We need to pass `--unsupported-gpu` as Sway checks for `nvidia` in
`/proc/modules`, and the Nix sandbox has `/proc/modules` available.
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.
When setting `...sway.package = null`, the default bar configuration
would throw an error trying to use the bar from the null package.
Logic is added to use the bar from `pkgs.sway` instead of `cfg.package`
if it is null.
Fixes#1714
