PostgreSQL external transaction feature

I have a big web application and tests which make requests to app running in sandbox. After each test I used to rollback database using db migrate rollback && db migrate && db seed. But now after test quantity rised, It takes much time. So, I am looking for feature which can wrap some amount of database command into a transaction and after test finish cancel transaction without modifying app source code (or make this by another way). May be there are some postgres database parameters or extensions?