Replace some raw nc usage in statschannel system test with curl [v9.18]

For tests where the TCP connection might get interrupted abruptly, replace the nc with curl as the data sent from server to client might get lost because of abrupt TCP connection. This happens when the TCP connection gets closed during sending the large request to the server.

As we already require curl for other system tests, replace the nc usage in the statschannel test with curl that actually understands the HTTP/1.1 protocol, so the same connection is reused for sending the consequtive requests, but without client-side "pipelining".

For the record, the server doesn't support parallel processing of the pipelined request, so it's a bit misnomer here, because what we are actually testing is that we process all requests received in a single TCP read callback.

Backport of !6946 (merged)

Closes #3270 (closed)