test: fix flaky test-http-set-timeout-server

It can happen that the connection and server is closed before the second
reponse has been processed by server. In this case, the
`res.setTimeout()` callback will never be called causing the test to
fail. Fix this by only closing the connection and server when the 2nd
has been received.

PR-URL: https://github.com/nodejs/node/pull/11790
Fixes: https://github.com/nodejs/node/issues/11768


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>