Drupal 9 compatability

Hi @jonathanshaw
Currently, The QueuingProcessingTest and AdvancedQueuingProcessingTest are skipped in tests. These ones need deep refactoring and should be mocking the ItemManager in the way that other tests are, not mocking the Xero client. Also, I didn't find a solution for the last usage (at(0) in the expectUpdateItemEventDispatched() function). I tried to use once(), and any() with code refactoring but I noticed that the dispatch method is called differently in various tests and the test has failures with once() or any() instead of at(0). Probably this function should be split for various tests. The deprecated method at() will be removed in PHPUnit 10, but it is working in PHPUnit 9.

Thanks @smovs, merged!

Automatically closed - issue fixed for 2 weeks with no activity.