nix: add graphical-gnome-test

This VM integration test spawns Gnome, monado-service,
stardust-xr-server, flatland and weston-cliptest and tests that the
functionality works correctly. The result is a screenshot. If any
program in the test produces an exit code above 0 it will fail the test,
graphical rendering bugs should be visible in the resulting screenshot

README.md

@@ -19,4 +19,27 @@ cargo build
 ## Install
 ```bash
 cargo install
-```
+```
+
+## Test
+
+##### Gnome Graphical Integration Test
+
+- `nix build .#gnome-graphical-test`
+
+   This test uses Nix to reproducibly execute a QEMU virtual machine which
+   spawns a full Gnome desktop. It runs `monado-service`, `stardust-xr-server`
+   `flatland` underneath of Gnome and then attaches `weston-cliptest` to the
+   `flatland` process running underneath of `stardust-xr-server`, the result is
+   a screenshot in PNG format that should look like expected. If any process in
+   this test produces an exit code above 0, the test will fail, graphical bugs
+   should be visible in the screenshot. An example of the result is below.
+
+   ###### Result
+
+   ![image](https://github.com/StardustXR/server/assets/26458780/e21cd039-2528-4568-b20a-ce4abfab6d9b)
+
+##### Everything
+
+`nix flake check` will build every test underneath of the `checks` attribute in the `flake.nix`
+