As per stackoverflow.com/a/52351480/895245 our standard test setup is:
while true; do
  resp=$"$(date): hello\n"
  len="$(printf '%s' "$resp" | wc -c)"
  printf "HTTP/1.1 200 OK\r\nContent-Length: $len\r\n\r\n${resp}\n" | nc -Nl 8000
done