test: update tests, migrate backend to vitest

Backend had tests written in mocha as well as a test written for jest
(created by an AI utility, wasn't compatible with mocha tests). The AI
generated test had merit so it was migrated to vitest along with all the
mocha tests, which supports conventions from both frameworks.

Vitest also has an excellent watcher cli.

The root repo package.json now runs unit tests in backend. Before it was
only running tests defined in the _test method of backend services.

package.json

@@ -28,7 +28,7 @@
     "webpack-cli": "^5.1.1"
   },
   "scripts": {
-    "test": "npx mocha src/phoenix/test && node src/backend/tools/test",
+    "test": "npx mocha src/phoenix/test && npx vitest run src/backend && node src/backend/tools/test",
     "start=gui": "nodemon --exec \"node dev-server.js\" ",
     "start": "node ./tools/run-selfhosted.js",
     "build": "cd src/gui; node ./build.js",