pkg/testutil: ForceGosched -> WaitSchedule

ForceGosched() performs bad when GOMAXPROCS>1. When GOMAXPROCS=1, it
could promise that other goroutines run long enough
because it always yield the processor to other goroutines. But it cannot
yield processor to goroutine running on other processors. So when
GOMAXPROCS>1, the yield may finish when goroutine on the other
processor just runs for little time.

Here is a test to confirm the case:

```
package main

import (
	"fmt"
	"runtime"
	"testing"
)

func ForceGosched() {
	// possibility enough to sched up to 10 go routines.
	for i := 0; i < 10000; i++ {
		runtime.Gosched()
	}
}

var d int

func loop(c chan struct{}) {
	for {
		select {
		case <-c:
			for i := 0; i < 1000; i++ {
				fmt.Sprintf("come to time %d", i)
			}
			d++
		}
	}
}

func TestLoop(t *testing.T) {
	c := make(chan struct{}, 1)
	go loop(c)
	c <- struct{}{}
	ForceGosched()
	if d != 1 {
		t.Fatal("d is not incremented")
	}
}
```

`go test -v -race` runs well, but `GOMAXPROCS=2 go test -v -race` fails.

Change the functionality to waiting for schedule to happen.

pkg/testutil/testutil.go

@@ -16,17 +16,13 @@ package testutil
 
 import (
 	"net/url"
-	"runtime"
 	"testing"
+	"time"
 )
 
-// WARNING: This is a hack.
-// Remove this when we are able to block/check the status of the go-routines.
-func ForceGosched() {
-	// possibility enough to sched up to 10 go routines.
-	for i := 0; i < 10000; i++ {
-		runtime.Gosched()
-	}
+// TODO: improve this when we are able to know the schedule or status of target go-routine.
+func WaitSchedule() {
+	time.Sleep(3 * time.Millisecond)
 }
 
 func MustNewURLs(t *testing.T, urls []string) []url.URL {