runtime: adjust TestPhysicalMemoryUtilization to handle large page sizes

Currently TestPhysicalMemoryUtilization can fail on systems with large
physical page sizes like 64 KiB because all the of the holes to be
scavenged are not aligned to the page size. The holes themselves are 64
KiB so this is actually quite likely.

Bump the size of the allocations for systems with larger physical page
sizes, and add additional slack to the threshold for unaligned pieces of
the holes that may be unaligned.

Fixes #49411.

Change-Id: Iafb35b8761dc9cdc53d3745c4771b1a64c5c97b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/363415


Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>