If stdin is noninteractive, the io.Copy from stdin to remoteStdin will
unblock when it finishes reading from stdin. In this case, make sure to
close remoteStdin so the server knows the client won't be sending any
more data. This ensures that the remote process terminates. For example:

echo foo | kubectl exec -i <pod> -- cat

Without this change, the `cat` process never terminates and `kubectl
exec` hangs.

Fix interactive exec sessions hanging after you type 'exit'.

Add e2e test to cover noninteractive stdin: `echo a | kubectl exec -i <pod>
cat`

Add e2e test to cover psuedo-interactive stdin: `kubectl exec -i <pod> bash`

Prep for sending multiple data frames over multiple streams in remote command
test, which is more likely to find flakes (requires bump of spdystream
once an issue with the frame worker queues not being fully drained when
a goaway frame is received).