sync: deflake TestWaitGroupMisuse3

If one of the helper goroutine panics, the main goroutine call to Wait
may hang forever waiting for something to call Done. Put that call in
a goroutine like the others.

Fixes #35774

Change-Id: I8d2b58d8f473644a49a95338f70111d4e6ed4e12
Reviewed-on: https://go-review.googlesource.com/c/go/+/210218
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/sync/waitgroup_test.go

@@ -147,7 +147,7 @@ func TestWaitGroupMisuse3(t *testing.T) {
 		}
 	}()
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(4))
-	done := make(chan interface{}, 2)
+	done := make(chan interface{}, 3)
 	// 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++ {
@@ -171,8 +171,13 @@ func TestWaitGroupMisuse3(t *testing.T) {
 			}()
 			wg.Wait()
 		}()
-		wg.Wait()
-		for j := 0; j < 2; j++ {
+		go func() {
+			defer func() {
+				done <- recover()
+			}()
+			wg.Wait()
+		}()
+		for j := 0; j < 3; j++ {
 			if err := <-done; err != nil {
 				panic(err)
 			}