time: gracefully handle ts.zombies underflow

time: gracefully handle ts.zombies underflow

The current implementation sets t.ts before adding t to ts;
that can cause inconsistencies with temporarily negative
ts.zombies values. Handle them gracefully, since we only
care about detecting very positive values.

Pending CL 564977 removes the race that sets t.ts early,
and then CL 569996 builds on top of that to make the count precise.
This CL just gets examples like the new test working sooner.

Change-Id: Ibe1aecc2554f83436f761f48e4050bd962982e4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/569995
Reviewed-by: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>