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: nodejs#19856
Fixes: nodejs#19804
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Rich Trott <[email protected]>

Author: santigimeno