fix(test): correct wrong offsets in mock Consumer

[grongor]

I tested it with Kafka console producer/consumer and I think that this change is correct.

$ kafka-console-consumer --bootstrap-server ********** --version
6.2.1-ce (Commit:06652ad189d0cbf0)

$ kafka-console-consumer --bootstrap-server ********** --topic tmp.dummy --from-beginning --max-messages 1 --property print.offset=true
[2021-12-04 15:24:27,631] WARN [Consumer clientId=consumer-console-consumer-36100-1, groupId=console-consumer-36100] Error while fetching metadata with correlation id 2 : {tmp.dummy=LEADER_NOT_AVAILABLE} (org.apache.kafka.clients.NetworkClient)
^CProcessed a total of 0 messages

$ kafka-console-consumer --bootstrap-server ********** --topic tmp.dummy --from-beginning --max-messages 1 --property print.offset=true
^CProcessed a total of 0 messages

$ kafka-console-producer --bootstrap-server ********** --topic tmp.dummy <<< "hello"

$ kafka-console-consumer --bootstrap-server ********** --topic tmp.dummy --from-beginning --max-messages 1 --property print.offset=true
Offset:0	hello
Processed a total of 1 messages

[dnwe]

@bai this is strange, I rebased the branch on main and for some reason the CLA tool is flagging up a failure, but I just merged a bunch of other PRs from @grongor that had the same GIT_AUTHOR and seemed to have passed the CLA check

[grongor]

So I tried signing again, but can't :/ At least I rebased the PR on the current master and fixed the fmt error.

[bai]

Thanks again for contributing (and late happy hols)! ❤️