Fix GroupChangesTestSuite.testGroupChangesOnSyncRoot randomly failing with PostgreSQL

The fix made for NXP-30399, replacing the forEach loop in NuxeoDriveGroupUpdateListener by a bulk action, introduced a random test error in GroupChangesTestSuite.testGroupChangesOnSyncRoot:

java.lang.AssertionError: expected:<1> but was:<0>
	at org.junit.Assert.fail(Assert.java:89)
	at org.junit.Assert.failNotEquals(Assert.java:835)
	at org.junit.Assert.assertEquals(Assert.java:647)
	at org.junit.Assert.assertEquals(Assert.java:633)
	at org.nuxeo.drive.fixtures.GroupChangesTestSuite.testGroupChanges(GroupChangesTestSuite.java:312)
	at org.nuxeo.drive.fixtures.GroupChangesTestSuite.testGroupChangesOnSyncRoot(GroupChangesTestSuite.java:160)

This is probably because the call to TransactionalFeature#nextTransaction() waits for the NuxeoDriveGroupUpdateListener async listener and the subsequent FireGroupUpdatedEventAction bulk action, but not always for the NuxeoDriveVirtualEventLogger async listener triggered afterwards.

Indeed, the call sequence is quite complex:

NuxeoDriveGroupUpdateListener (async)
  -> FireGroupUpdatedEventAction
    -> NuxeoDriveFileSystemDeletionListener (sync)
      -> NuxeoDriveVirtualEventLogger (async)
        -> AuditLogger#addLogEntries

A possible solution is to double the calls to TransactionalFeature#nextTransaction() in GroupChangesTestSuite#commitAndWaitForAsyncCompletion().

In the future, we'll want to avoid having such a stack of async processing as it's very difficult to analyze and test.
BAF triggering WorkManager is not a good idea.
Using BAF everywhere should solve the problem.