runtime: relax TestWindowsStackMemory from 100kB to 128kB

runtime: relax TestWindowsStackMemory from 100kB to 128kB

We've been getting intermittent flakes in this test since 2023,
all reporting values just barely over 100kB on windows-386.

If we were happy with 100kB, we should be happy with 128kB,
and it should fix the flakes.

Fixes #58570.

Change-Id: Iabe734cfbba6fe28a83f62e7811ee03fed424f0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/628795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>