rate: extend maximum-delay slop to all platforms

I initially added some timing slop for specific platforms based on the
failures we had seen so far (and a theory explaining at least some of
those failures). But now we've seen overruns on other platforms too,
so perhaps we should use the same factors everywhere.

Fixes #43055 (I hope).

Change-Id: Ib6035b55acd9d17acc7ad78065abf5f1e3f9dfe7
Reviewed-on: https://go-review.googlesource.com/c/time/+/399535
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>