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