chore: cover first node:http proxy request under test (#35001)
Adds a regression spec for the node:http proxy case where the first
request under `deno test` could receive a generated 400 while the nested
outgoing request never reached the target.
The spec runs through `deno test` itself and asserts both the first and
second raw proxy requests return 200 and reach the target server.
Closes #28261
Closes denoland/divybot#539
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Co-authored-by: divybot <divybot@users.noreply.github.com>
Co-authored-by: Divy Srivastava <me@littledivy.com>
