Investigate the problem with the flaky behaviour of the MultiSubscriptionFetcherTest in the CI/CD

[DawidNiezgodka]

For some reason, the tests in the MultiSubscriptionFetcherTest class don't always behave as expected. The order of the consumed records provided by _SubscriptionProvider_s is not deterministic. This can cause the tests to fail as we check the final result assuming a specific order of records.
In this issue we want to investigate the problem and find the reason behind it. Some preliminary presumption are: race conditions in the reactive library or ordering of map.

[DawidNiezgodka]

After first examination, the fields in the MultiSubscriptionFetcher code are processed in the correct order (that is, the ConsumerRecords from the SubscriptionProviders).
It seems the problem occurs when issuing items from the map to the subscriber, i.e. when mapPublisher.subscribe and then testSubs.assertValueAt are called.
When one creates a TestSubscriber with a custom implementation of the Subscriber, which writes Items from onNext in the console, then one can already see that the order of onNext is already different than it is desired.