I can reproduce with a very short timeout (fractions of a millisecond)
combined with -race.

But given that this is inherently sensitive to actual time, add a
testing mechanism to retry with increasingly large times to compensate
for busy buidlers. This also means the test is usually faster now,
too, since we can start with smaller durations.

Fixes #19608

Change-Id: I3a222464720195849da768e9801eb7b43baa4aeb
Reviewed-on: https://go-review.googlesource.com/82595
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>