net/http: improve test leak checker flakiness

And make it have more useful output on failure.

Update #5005

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8016046

src/pkg/net/http/z_last_test.go

@@ -65,8 +65,6 @@ func afterTest(t *testing.T) {
 	if testing.Short() {
 		return
 	}
-	buf := make([]byte, 1<<20)
-	var stacks string
 	var bad string
 	badSubstring := map[string]string{
 		").readLoop(":                                  "a Transport",
@@ -75,9 +73,10 @@ func afterTest(t *testing.T) {
 		"timeoutHandler":                               "a TimeoutHandler",
 		"net.(*netFD).connect(":                        "a timing out dial",
 	}
+	var stacks string
 	for i := 0; i < 4; i++ {
 		bad = ""
-		stacks = string(buf[:runtime.Stack(buf, true)])
+		stacks = strings.Join(interestingGoroutines(), "\n\n")
 		for substr, what := range badSubstring {
 			if strings.Contains(stacks, substr) {
 				bad = what
@@ -90,6 +89,5 @@ func afterTest(t *testing.T) {
 		// shutting down, so give it some time.
 		time.Sleep(250 * time.Millisecond)
 	}
-	gs := interestingGoroutines()
-	t.Errorf("Test appears to have leaked %s:\n%s", bad, strings.Join(gs, "\n\n"))
+	t.Errorf("Test appears to have leaked %s:\n%s", bad, stacks)
 }