fix: Disable threads for project-sidebar-internal

Part of request #27556: Migrate Vite-based packages to Vitest

project-sidebar-internal's JS unit tests get stuck from time to time.
It's been reproduced locally but it does not trigger always. An issue
comment [0] on vitest's github advises to disable threads to mitigate
the issue. We are going to try that.

[0]:
vitest-dev/vitest#2008 (comment)

Change-Id: I4ae7344745b4f4d8ac30b5a21113e461690d0d5e

lib/frontend/project-sidebar-internal/package.json

@@ -31,6 +31,6 @@
     "typecheck": "vue-tsc --noEmit",
     "build": "vite build",
     "dev": "vite",
-    "test": "vitest"
+    "test": "vitest --no-threads"
   }
 }