[build] Add a property to specify a custom timeout

This allows to later reduce the default timeout, once all outliers use a
custom timeout.

Change-Id: I22ec2d816423f309b15c37f9b37e167749dce4ba
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/173972
Auto-Submit: Alexander Thomas <athom@google.com>
Reviewed-by: Karl Klose <karlklose@google.com>
Commit-Queue: Karl Klose <karlklose@google.com>
5 files changed