test(cloudflare): Add integration test infrastructure

[timfish]

• cc @0xbad0c0d3 who's been adding tests

I tried using cloudflare:test and the wrangler vitest pool but with the config it doesn't appear like there's an easy way to have more than one wrangler config. This isn't very useful if you want to add lots of different tests with different wrangler configurations!

After much frustration and attempts I decided to just copy the Node integration test runner and strip out all the stuff we didn't need. This took less time than I'd already spend messing around. I debated making the test runners use shared code but quickly discounted that because of how little actually ends up being shared.

This PR:

• Moves createBasicSentryServer from the Node integration test to @sentry-internal/test-utils so it can be used by both
• Added dev-packages/cloudflare-integration-tests/ with all the basic files it needs
• Create a simplified test runner that runs wrangler in dev mode
  • spawn('wrangler', ['dev', '--config', wranglerConfigPath, '--var', 'SENTRY_DSN:${SENTRY_DSN}']);
• Adds a simple test for throwing in the fetch handler which asserts the entire envelope
• Adds the new test project to CI