test: make issue8606b test more robust
test: make issue8606b test more robust

Use actual unmapped memory instead of small integers to make
pointers that will fault when accessed.

Fixes #49562

Change-Id: I2c60c97cf80494dd962a07d10cfeaff6a00f4f8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/364914
Trust: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Run-TryBot: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

golang/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,golang/go,golang/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go