Use Custom Prefix for Redis Tests

[sbernard31]

In Integration tests we have several Redis tests.
Theoretically, each tests should clean key added in Redis but would be cleaner to use a dedicated prefix.
So dev can clean key related to tests easily.

In code we should have some comments like :

        // Create redis store
        Pool<Jedis> jedis = RedisTestUtil.createJedisPool();
        // TODO use custom key when https://github.com/eclipse/leshan/issues/1249 will be available
        builder.setRegistrationStore(new RedisRegistrationStore(jedis));