Fix Tauri testing (#6462)

# Description of Changes
#6402 introduced a Rust test `refresh_token_fallback.rs`, but it wasn't
moved properly after the restructure of the `frontend/` folder in #6404.
This PR moves the file to the right place, and also hooks up Task and CI
rules for `cargo test` since nothing was actually running the test in
the first place.

.taskfiles/desktop.yml

@@ -78,6 +78,13 @@ tasks:
     cmds:
       - npx tauri build --bundles appimage
 
+  test:
+    desc: "Run Tauri/Cargo tests"
+    deps: [prepare]
+    dir: editor/src-tauri
+    cmds:
+      - cargo test
+
   clean:
     desc: "Clean Tauri/Cargo build artifacts"
     dir: editor