Adding two messages into a group may end up with wrong insert order.
Or better to say the timestamp of first messages inserted might be behind in the future after the second one.
This leads to a wrong order return for polling operation and, therefore, verification in the test.
* Fix `PostgresChannelMessageTableSubscriberTests` inserting `Thread.sleep(100);`
between `messageStore.addMessageToGroup()` operations
32e3d18c94