RedisTest throws \RedisException, so expect that, not Warning

[DavidGoodwin]

RedisTest errors with :

1) Stash\Test\Driver\RedisTest::testBadDisconnect
PHPUnit\Framework\Exception: Class "Warning" does not exist

Caused by
ReflectionException: Class "Warning" does not exist

Fix by changing class to be RedisException which does seem to be thrown.