runtime: only run TestMemStats sanity tests once

Fixes #22696

Change-Id: Ibe4628f71d64a2b36b655ea69710a925924b12a3
Reviewed-on: https://go-review.googlesource.com/122586
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/malloc_test.go

@@ -19,7 +19,11 @@ import (
 	"unsafe"
 )
 
+var testMemStatsCount int
+
 func TestMemStats(t *testing.T) {
+	testMemStatsCount++
+
 	// Make sure there's at least one forced GC.
 	GC()
 
@@ -35,6 +39,13 @@ func TestMemStats(t *testing.T) {
 	}
 	le := func(thresh float64) func(interface{}) error {
 		return func(x interface{}) error {
+			// These sanity tests aren't necessarily valid
+			// with high -test.count values, so only run
+			// them once.
+			if testMemStatsCount > 1 {
+				return nil
+			}
+
 			if reflect.ValueOf(x).Convert(reflect.TypeOf(thresh)).Float() < thresh {
 				return nil
 			}