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.

.github/workflows/tauri-build.yml

@@ -157,6 +157,9 @@ jobs:
           JPDFIUM_PLATFORMS: ${{ matrix.jpdfium_platforms }}
         run: task desktop:prepare
 
+      - name: Run Tauri/Cargo tests
+        run: task desktop:test
+
       # DigiCert KeyLocker Setup (Cloud HSM)
       - name: Setup DigiCert KeyLocker
         id: digicert-setup

.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