net: allow more generous slop in Fluctuation tests

It appears that at least the OpenBSD kernel gets sloppier the longer
the timeout we give it, up to an observed overhead of around 25%.
Let's give it a little more than that (33%) in the comparison, and
also increase the growth curve to match the actual observed times
instead of exponential initial growth.

Fixes #36108

Change-Id: Id3e54559b7c45b7c8bc0ca07dce74ca60e77e7ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/366176


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>