Add test to check that only backend files with correct prefix are deleted.
Add test to check that only backend files with correct prefix are deleted.

This concerns issue #2678.

duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati