http: make timeout event work with agent timeout

The `'timeout'` event is currently not emitted on the `ClientRequest`
instance when the socket timeout expires if only the `timeout` option
of the agent is set. This happens because, under these circumstances,
`listenSocketTimeout()` is not called.

This commit fixes the issue by calling it also when only the agent
`timeout` option is set.

PR-URL: https://github.com/nodejs/node/pull/25488


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>