engine: the flow test fixture names its backend

`FluidStats` gained `backend` in #535 and the fixture of the flow drawing
tests arrived with #550; merged one after the other, the fixture lacks the
field and `cargo test -p fieldcraft-engine` no longer compiles on develop.

Part of #157.

crates/engine/src/flow.rs

@@ -437,6 +437,7 @@ pub(crate) mod tests {
             lattice_velocity: 0.1,
             mach: 0.1,
             cell_updates: 0,
+            backend: "cpu".into(),
         };
         FluidResult {
             step: "Flow".into(),