hash/maphash: weaken avalanche test a bit

hash/maphash: weaken avalanche test a bit

Give the test a bit more wiggle room.

Previously the allowed range was about 46.5% to 53.5%. Now it is about 43% TO 57%.

Fixes #60170

Change-Id: Ieda471e0986c52edb9f6d31beb8e41917876d6c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/495415
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>