Port some internal streams tests to wd-test format

[jasnell]

Had claude port a handful of the easier-to-port tests and provide a checklist for doing the rest. Some will require manual figuring out to port, most likely can be automated with claude.

PR is tests and doc only. No functional changes. As long as CI passes they should be good. Once existing tests have been ported, a separate analysis of test coverage will be performed to determine what is missing.

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Current state visible to Cloudflare employees: https://docs.google.com/document/d/178N1WCas0PLO4nLnr597daR5XI8NRo3lRQW3G7mOvHI/edit?tab=t.0

[anonrig]

Did you validate that these are 1 to 1 replica of the ew-tests? It's hard to review. Knowing that you've validated will ease the review process.

[jasnell]

@anonrig ...Please go ahead with whatever additional changes you feel are necessary here. Feel free to push additional commits. Before you do so, it might be easier to merge the other PR into this one to save some rebasing trouble later.

[jasnell]

This likely needs a bit more of a looking at to make sure all of the ported tests are actually from ew-tests and not just incorrectly copied from already existing workerd tests.

[harrishancock]

I've gotten partway through pipe-streams-test, but need to give my brain a rest.

Most of my comments so far are non-blocking observations or nits, I think, but there are a few real issues.

[jasnell]

For those reviewing.. just as an FYI... @anonrig has taken this PR over.

[anonrig]

Fixing conflicts before addressing the comments.

[anonrig]

@guybedford @harrishancock would you mind reviewing this? i'd like to land this asap so that we can iteratively improve the test coverage.