After socket.setEncoding(enc), socket.pipe(destination) wrote the raw
bytes it read to the destination. It now writes the decoded strings, as
Node's pipe writes each string chunk: the destination receives their
UTF-8 bytes, and a partial character the decoder holds at EOF is written
before the destination ends.
tests/node-parallel/run.mjs runs each job's files with its own
TEST_THREAD_ID, as Node's test runner does, so test/common/tmpdir gives
concurrent files different directories instead of one shared .tmp.0.
