Run e2e tests sequentially (#2012)

This instructs vitest to run all e2e tests sequentially.

Without this, vitest may run several spec files at the same time, which
seems to cause issues with the virtual authenticator, leading to
flakiness.

package.json

@@ -22,7 +22,7 @@
     "preview:showcase": "astro preview --root ./src/showcase",
     "screenshots": "npm run opts -- ./src/frontend/screenshots.ts",
     "test": "vitest --config ./vitest.config.ts",
-    "test:e2e": "vitest --mode e2e --config ./vitest.config.ts",
+    "test:e2e": "vitest --mode e2e --config ./vitest.config.ts --threads=false",
     "test:e2e-desktop": "SCREEN=desktop npm run test:e2e",
     "test:e2e-mobile": "SCREEN=mobile npm run test:e2e",
     "lint": "eslint --max-warnings 0 --cache --cache-location node_modules/.cache/eslint 'src/frontend/**/*.ts*' 'src/showcase/**/*.ts'",