chore: investigate fixtures tests flakiness

[petebacondarwin]

• [ ] consider using the file-based Worker registry for these tests
• [ ] diagnose what is causing tests to timeout (workerd/Miniflare crashes?)
• [ ] prune fixture tests that don't add any value from the CI

[petebacondarwin]

An example of such a flaky test is the get-platform-proxy fixture:

   ❯ tests/get-platform-proxy.env.test.ts  (18 tests) 25004ms

  ⎯⎯⎯⎯⎯⎯ Failed Suites 1 ⎯⎯⎯⎯⎯⎯⎯

   FAIL  tests/get-platform-proxy.env.test.ts > getPlatformProxy - env
  Error: Hook timed out in 25000ms.
  If this is a long-running hook, pass a timeout value as the last argument or configure it globally with "hookTimeout".
  ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯[1/2]⎯

   FAIL  tests/get-platform-proxy.env.test.ts > getPlatformProxy - env
  TypeError: Cannot read properties of undefined (reading 'map')
   ❯ tests/get-platform-proxy.env.test.ts:55:39
       53| 
       54|  afterAll(async () => {
       55|   await Promise.allSettled(devWorkers.map((i) => i.stop()));
         |                                       ^
       56|  });
       57| 

  ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯[2/2]⎯