runtime: randomize order of timers at the same instant in bubbles
In synctest bubbles, fire timers scheduled for the same instant
in a randomized order.
Pending timers are added to a heap ordered by the timer's wakeup time.
Add a per-timer random value, set when the timer is added to a heap,
to break ties between timers scheduled for the same instant.
Only inject this randomness in synctest bubbles. We could do so
for all timers at the cost of one cheaprand call per timer,
but given that it's effectively impossible to create two timers
scheduled for the same instant outside of a fake-time environment,
don't bother.
Fixes #73876
For #73850
Change-Id: Ie96c86a816f548d4c31e4e014bf9293639155bd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/677276
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>