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>

src/runtime/export_test.go

@@ -297,6 +297,7 @@ func ReadMemStatsSlow() (base, slow MemStats) {
 		slow.TotalAlloc = 0
 		slow.Mallocs = 0
 		slow.Frees = 0
+		slow.HeapReleased = 0
 		var bySize [_NumSizeClasses]struct {
 			Mallocs, Frees uint64
 		}
@@ -336,6 +337,10 @@ func ReadMemStatsSlow() (base, slow MemStats) {
 			slow.BySize[i].Frees = bySize[i].Frees
 		}
 
+		mheap_.scav.treap.walkTreap(func(tn *treapNode) {
+			slow.HeapReleased += uint64(tn.spanKey.released())
+		})
+
 		getg().m.mallocing--
 	})