fix trash bin expire operation and add unit tests

add unit tests for expire operation.
Not sure. The expire method is hard to test, reproduce. Because you need files with a specific age or you need to run out of space. So if we become bug reports it's always hard to say if everything was OK or if something went wrong. Therefore I like to have this logs, because then the owner can easily see why the files are deleted and if it was the right decision at the time it happened.