Run tests with --serial

...to try and improve test stability. This isn't ideal, but hopefully
it should fix some of the flakiness we're seeing. We can investigate
this further later.

Author: mrbbot

package.json

@@ -27,7 +27,7 @@
     "lint:fix": "npm run lint -- --fix",
     "prepublishOnly": "npm run lint && npm run clean && npm run build && npm run types:bundle && npm run test",
     "release": "./scripts/release.sh",
-    "test": "npm run build && ava && rimraf ./.tmp",
+    "test": "npm run build && ava --serial && rimraf ./.tmp",
     "types:build": "tsc && tsc -p packages/miniflare/src/workers/tsconfig.json",
     "types:bundle": "npm run types:build && node scripts/types.mjs"
   },