Fix for first run fail of slowlog test

Have added a fix for first run fail of the slowlog tests.

Please refer to the [commit][1]  for the fix.

@DTrejo  @brycebaril there was a minor glitch in the test code, that caused the test to fail in the first attempt. I had a running server so i was not able repro this issue before. Thanks @brycebaril for pointing out.


[1]:https://github.com/nitesh123/node_redis/commit/79ea20391ec6ccb51ab986d6204f2285191b29f5