test: move http-client-timeout-agent to sequential

Move resource-sensitive test from parallel to sequential.

Fixes: https://github.com/nodejs/node/issues/19804

PR-URL: https://github.com/nodejs/node/pull/19809


Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>