Rollup merge of #142654 - workingjubilee:increase-timeout-factor-on-mpmc-test, r=joshtriplett

library: Increase timeout on mpmc test to reduce flakes

This recently spuriously failed in a rollup, so I think we can afford to increase the base timeout and the amount of time slept for to provide a much wider margin for the timeout to be reached.

Author: tgross35