runtime: extend ReadMemStatsSlow to re-compute HeapReleased

runtime: extend ReadMemStatsSlow to re-compute HeapReleased

This change extends the test function ReadMemStatsSlow to re-compute
the HeapReleased statistic such that it is checked in testing to be
consistent with the bookkeeping done in the runtime.

Change-Id: I49f5c2620f5731edea8e9f768744cf997dcd7c22
Reviewed-on: https://go-review.googlesource.com/c/142397
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>