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.

rafthttp/pipeline_test.go

@@ -39,7 +39,7 @@ func TestPipelineSend(t *testing.T) {
 	p := newPipeline(tr, picker, types.ID(2), types.ID(1), types.ID(1), fs, &fakeRaft{}, nil)
 
 	p.msgc <- raftpb.Message{Type: raftpb.MsgApp}
-	testutil.ForceGosched()
+	testutil.WaitSchedule()
 	p.stop()
 
 	if tr.Request() == nil {
@@ -60,7 +60,7 @@ func TestPipelineExceedMaximalServing(t *testing.T) {
 
 	// keep the sender busy and make the buffer full
 	// nothing can go out as we block the sender
-	testutil.ForceGosched()
+	testutil.WaitSchedule()
 	for i := 0; i < connPerPipeline+pipelineBufSize; i++ {
 		select {
 		case p.msgc <- raftpb.Message{}:
@@ -68,7 +68,7 @@ func TestPipelineExceedMaximalServing(t *testing.T) {
 			t.Errorf("failed to send out message")
 		}
 		// force the sender to grab data
-		testutil.ForceGosched()
+		testutil.WaitSchedule()
 	}
 
 	// try to send a data when we are sure the buffer is full
@@ -80,7 +80,7 @@ func TestPipelineExceedMaximalServing(t *testing.T) {
 
 	// unblock the senders and force them to send out the data
 	tr.unblock()
-	testutil.ForceGosched()
+	testutil.WaitSchedule()
 
 	// It could send new data after previous ones succeed
 	select {
@@ -99,7 +99,7 @@ func TestPipelineSendFailed(t *testing.T) {
 	p := newPipeline(newRespRoundTripper(0, errors.New("blah")), picker, types.ID(2), types.ID(1), types.ID(1), fs, &fakeRaft{}, nil)
 
 	p.msgc <- raftpb.Message{Type: raftpb.MsgApp}
-	testutil.ForceGosched()
+	testutil.WaitSchedule()
 	p.stop()
 
 	fs.Lock()