runtime: rewrite TestPhysicalMemoryUtilization
runtime: rewrite TestPhysicalMemoryUtilization

This test changes TestPhysicalMemoryUtilization to be simpler, more
robust, and more honest about what's going on.

Fixes #49411.

Change-Id: I913ef055c6e166c104c62595c1597d44db62018c
Reviewed-on: https://go-review.googlesource.com/c/go/+/362978
Trust: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>

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