Additional meta-testing for the Workers Vitest integration

[mrbbot]

Currently, the Vitest integration is tested against a suite of example tests. There are some things that can't be tested this way though:

• [x] (Inline) snapshots, updating existing and remove obsolete
• [x] console.log()ing (including inside SELF) with source maps
• [x] Watch mode (for unit tests and SELF integration tests https://github.com/cloudflare/workers-sdk/issues/5384)
• [x] Config validation and pretty-formatting of validation errors
• [x] Concurrent tests failing with isolated storage enabled
• [x] Ensuring required compatibility flags set
• [x] Service worker main detection and failure
• [ ] Dynamic import() (for unit tests and SELF integration tests https://github.com/cloudflare/workers-sdk/issues/5387)
• [x] Large modules chunking

These require us to (re)start Vitest programatically and assert on its output, or interact with a running session. We'll also want to make sure we're testing...

• [x] Fake timers
• [x] Different isolation models
• [x] Mock fallthrough to the internet

[petebacondarwin]

Closed by #5869