Fixes RedisCanBeAccessed inconsistent check results

[Namoshek]

This is only a small PR that hopefully fixes inconsistent check results of the RedisCanBeAccessed check. When I tested the foundation for this test, Redis::connection($name), it seemed to already return a connected connection. But my recent tests do not really confirm this. So I made the check a bit more expressive by explicitely connecting to the redis server.

I also added some additional test cases and additional error messages (also for better testability).

[slavarazum]

Have found that Redis::connection()->isConnected() always returns false if previously application didn't connect to it.
Try to connect at first in RedisCanBeAccessed before check by isConnected() method.

Tested on Ubuntu: http://prntscr.com/k97e9b
Tested on Mac: http://prntscr.com/k97eoi

Also it works if you try to get some key value: http://prntscr.com/k97j2a

[eduardokum]

You can check the connection this way too.

Str::upper(Redis::connection()->executeRaw(['ping'])) != 'PONG'

like this. eduardokum@9d06479