// IN_PROGRESS.SenderAccountTransactionObserver (DEFAULT)
// IN_PROGRESS.ReceiverAccountTransactionObserver (APPLICATION + 1)

The test assumes that an observer with priority DEFAULT is called before an observer with priority APPLICATION + 1.

In fact however,
DEFAULT = APPLICATION + 500 = 2500
APPLICATION + 1 = 2001

Therefore, the correct order is APPLICATION + 1, DEFAULT