sync: simplify TestOncePanic

Follow-up to CL 137350043.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/141620043

src/sync/once_test.go

@@ -40,26 +40,20 @@ func TestOnce(t *testing.T) {
 }
 
 func TestOncePanic(t *testing.T) {
-	once := new(Once)
-	for i := 0; i < 2; i++ {
+	var once Once
 	func() {
 		defer func() {
-				r := recover()
-				if r == nil && i == 0 {
-					t.Fatalf("Once.Do() has not panic'ed on first iteration")
-				}
-				if r != nil && i == 1 {
-					t.Fatalf("Once.Do() has panic'ed on second iteration")
+			if r := recover(); r == nil {
+				t.Fatalf("Once.Do did not panic")
 			}
 		}()
 		once.Do(func() {
 			panic("failed")
 		})
 	}()
-	}
-	once.Do(func() {})
+
 	once.Do(func() {
-		t.Fatalf("Once called twice")
+		t.Fatalf("Once.Do called twice")
 	})
 }