CI: portfwd tests should cover keepalive and other connectivity issues

[AkihiroSuda]

Thanks, let's merge this, but we should eventually run a test for this on CI

Originally posted by @AkihiroSuda in #3684 (review)

Should cover:

• Stuck network when using multiple connections with k3s #3601
• [gRPC portfwd] client connection is not closed immediately when server closed the connection #3685
  • CI: test and fix portfwd issue (w3m -dump hangs) #3708