tests: add explicit test to demo default to 0 db on redis-sentinel without db opt

asynq_test.go

@@ -146,6 +146,7 @@ func TestParseRedisURI(t *testing.T) {
 				MasterName:       "mymaster",
 				SentinelAddrs:    []string{"localhost:5000", "localhost:5001", "localhost:5002"},
 				SentinelPassword: "mypassword",
+				DB:               0,
 			},
 		},
 		{