runtime/pprof: run TestGoroutineSwitch for longer

Short test now takes about 0.5 second here.

Fixes #6417.

The failure was also seen on our builders.

R=golang-dev, minux.ma, r
CC=golang-dev
https://golang.org/cl/13321048

src/pkg/runtime/pprof/pprof_test.go

@@ -183,11 +183,11 @@ func TestCPUProfileWithFork(t *testing.T) {
 // If it did, it would see inconsistent state and would either record an incorrect stack
 // or crash because the stack was malformed.
 func TestGoroutineSwitch(t *testing.T) {
-	// How much to try. These defaults take about 1 seconds
-	// on a 2012 MacBook Pro. The ones in short mode take
-	// about 0.1 seconds.
+	// How much to try. These defaults take about 6 seconds
+	// on a 2011 Windows 7 64 bit notebook. The ones in short mode take
+	// about 0.6 seconds.
 	tries := 10
-	count := 1000000
+	count := 4000000
 	if testing.Short() {
 		tries = 1
 	}