Dont' assume SET ordering in tests

Small fix to test assertions for sadd/smembers. Simply checks the index of the member rather than assuming ordering on a SET, since they don't guarantee order and all.

Error used to be:

```shell
- sadd2:
Uncaught exception: AssertionError: "member2" === "member0"
    at tests.SADD2 (/Users/gjohnson/Projects/bitcrunch/node_modules/redis/test.js:998:16)
    at try_callback (/Users/gjohnson/Projects/bitcrunch/node_modules/redis/index.js:520:9)
    at RedisClient.return_reply (/Users/gjohnson/Projects/bitcrunch/node_modules/redis/index.js:590:13)
    at ReplyParser.RedisClient.init_parser (/Users/gjohnson/Projects/bitcrunch/node_modules/redis/index.js:263:14)
    at ReplyParser.EventEmitter.emit (events.js:96:17)
    at ReplyParser.send_reply (/Users/gjohnson/Projects/bitcrunch/node_modules/redis/lib/parser/javascript.js:297:10)
    at ReplyParser.execute (/Users/gjohnson/Projects/bitcrunch/node_modules/redis/lib/parser/javascript.js:211:22)
    at RedisClient.on_data (/Users/gjohnson/Projects/bitcrunch/node_modules/redis/index.js:476:27)
    at Socket.<anonymous> (/Users/gjohnson/Projects/bitcrunch/node_modules/redis/index.js:79:14)
    at Socket.EventEmitter.emit (events.js:96:17)
```