net/http/httptest: fix deadlock in TestIssue7264

I am seeing deadlocks waiting on <-inHandler.
It seems to me that there is no guarantee that the
handler actually runs, if the client does

        write header
        close connection

fast enough. The server might see the EOF on the
connection before it manages to invoke the handler.

This change fixes the deadlock, but it may make
the test not actually test anything. Not sure.

LGTM=bradfitz
R=bradfitz, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/140970043