Unit test needed for exercising attachment upload timeouts

[FrankHassanabad]

As per this PR and discussion:
#73103 (comment)

router.test.ts needs a unit test (if possible) to be able to timeout when doing an upload and showing off a 408 Request timeout or a socket hangup when the timeout is exceeded. The tests right now exercise the server side timing out after the payload from the PUT/POST has reached the server but we don't have an integration/unit test which shows the 408 Request timeout happening after a socket connection starts and then the upload delays or goes so slowly that the server disconnects and. gives a 408 or socket hangup when the timeout is small.

Kibana version:
7.9.0, 7.10.0

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)