fix: rpcenc: deflake TestReaderRedirectDrop

[magik6k]

Related Issues

Fixes #10201

Proposed Changes

• Make the test reproduce the original issue with near ~100% reliability
• Fix the edge-case
• Update the test to deal with racy behavior correctly

Additional Info

• beginPost "transforms" the virtual reader into a real one after the first call to Read
• TestReaderRedirectDrop tests what happens when the connection from the client to the redirecting node drops juuust before the redirecting node attempts to call a target node with a redirected reader
  • There appears to be a race between the redirected stream being accepted/consumed on the decoding end, and the redirect logic noticing that the stream that we have just tried to redirect was closed.
  • Fixing that would require a lot of hacky instrumentation, so for now the test accepts both, entirely correct, outcomes from the closed to-be-redirected head request.
• closeOnce needs to be propagated because it's what's guarding nr.res from double-close

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, market, mempool, multisig, networking, paych, proving, sealing, wallet, deps
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green