Support new CLIENT KILL semantics from Redis 2.8.12+ in tests

CLIENT KILL since version 2.8.12 has a new format:
```
CLIENT KILL <filter> <value> ... ... <filter> <value>
```
http://redis.io/commands/client-kill

The interesting part is `CLIENT KILL SKIPME yes/no`.
By default this option is set to yes, which means that something in the sense of client.kill(self) wouldn't work anymore.

This commit changes the test to use a different client for killing the desired connection, if the redis version is greater than 2.8.12

This does not, however, enable the usage of the new syntax. That is stuff for a new PR.