sync: deflake TestWaitGroupMisuse3

Previous flakes:
https://build.golang.org/log/223365dedb6b6aa0cfdf5afd0a50fd433a16bade
https://build.golang.org/log/edbea4cd3f24e707ef2ae8378559bb0fcc453c22

Dmitry says in email about this:

> The stack trace points to it pretty clearly. Done can indeed unblock
> Wait first and then panic. I guess we need to recover after first
> Done as well.

And it looks like TestWaitGroupMisuse2 was already hardened against
this.  Do the same in TestWaitGroupMisuse3.

Change-Id: I317800c7e46f13c97873f0873c759a489dd5f47d
Reviewed-on: https://go-review.googlesource.com/19183Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/sync/waitgroup_test.go

@@ -128,13 +128,16 @@ func TestWaitGroupMisuse3(t *testing.T) {
 		}
 	}()
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(4))
-	done := make(chan interface{}, 1)
+	done := make(chan interface{}, 2)
 	// The detection is opportunistically, so we want it to panic
 	// at least in one run out of a million.
 	for i := 0; i < 1e6; i++ {
 		var wg WaitGroup
 		wg.Add(1)
 		go func() {
+			defer func() {
+				done <- recover()
+			}()
 			wg.Done()
 		}()
 		go func() {
@@ -150,8 +153,10 @@ func TestWaitGroupMisuse3(t *testing.T) {
 			wg.Wait()
 		}()
 		wg.Wait()
-		if err := <-done; err != nil {
-			panic(err)
+		for j := 0; j < 2; j++ {
+			if err := <-done; err != nil {
+				panic(err)
+			}
 		}
 	}
 	t.Fatal("Should panic")