Syncronize cancellation in windows-restart tests

Two windows-restart tests would timeout and fail due to the cancellation
thread firing before the cancel object was created. This change syncronizes
the start of the threads to prevent this from occurring.

provisioner/windows-restart/provisioner_test.go

@@ -268,15 +268,18 @@ func TestProvision_waitForCommunicatorWithCancel(t *testing.T) {
 	// Run 2 goroutines;
 	//  1st to call waitForCommunicator (that will always fail)
 	//  2nd to cancel the operation
+	waitStart := make(chan bool)
 	waitDone := make(chan bool)
 	go func() {
+		waitStart <- true
 		err = waitForCommunicator(p)
+		waitDone <- true
 	}()
 
 	go func() {
 		time.Sleep(10 * time.Millisecond)
+		<-waitStart
 		p.Cancel()
-		waitDone <- true
 	}()
 	<-waitDone
 
@@ -327,13 +330,15 @@ func TestProvision_Cancel(t *testing.T) {
 
 	comm := new(packer.MockCommunicator)
 	p.Prepare(config)
+	waitStart := make(chan bool)
 	waitDone := make(chan bool)
 
 	// Block until cancel comes through
 	waitForCommunicator = func(p *Provisioner) error {
+		waitStart <- true
 		for {
 			select {
-			case <-waitDone:
+			case <-p.cancel:
 			}
 		}
 	}
@@ -346,6 +351,7 @@ func TestProvision_Cancel(t *testing.T) {
 	}()
 
 	go func() {
+		<-waitStart
 		p.Cancel()
 	}()
 	<-waitDone