When running the client header timeout test, there is a race between
us timing out and waiting on the remaining requests to be serviced. If
the client times out before the server blocks on the channel in the
handler, we will be simultaneously adding to a waitgroup with the
value 0 and waiting on it when we call TestServer.Close().

This is largely a theoretical race. We have to time out before we
enter the handler and the only reason we would time out if we're
blocked on the channel. Nevertheless, make the race detector happy
by turning the close into a channel send. This turns the defer call
into a synchronization point and we can be sure that we've entered
the handler before we close the server.

Fixes #10780

Change-Id: Id73b017d1eb7503e446aa51538712ef49f2f5c9e
Reviewed-on: https://go-review.googlesource.com/9905Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>