rusticl/device: skip loading devices in cfg(test)

Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35188>

src/gallium/frontends/rusticl/core/device.rs

@@ -1333,8 +1333,12 @@ impl Device {
     }
 
     pub fn all() -> Vec<Device> {
+        #[cfg(not(test))]
         let mut devs: Vec<_> = load_screens().filter_map(Device::new).collect();
 
+        #[cfg(test)]
+        let mut devs: Vec<Device> = Vec::default();
+
         // Pick a default device. One must be the default one no matter what. And custom devices can
         // only be that one if they are the only devices available.
         //