test: fix flaky http-client-timeout-agent

Close the connection immediately after receiving the `end` event to
avoid that the `timeout` event is fired before the connection closes
gracefully.
Move the test back to parallel.

PR-URL: https://github.com/nodejs/node/pull/19856
Fixes: https://github.com/nodejs/node/issues/19804


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>