The database engine is designed for continuous use in real applications. If you have an application that uses lobs and deletes some of them, the space will be reused for future lobs after each checkpoint.
In normal application use, the DELETE statement is used to delete rows. This statement deallocates the lob space for reuse after each checkpoint.
You can design your tests in a way that recreates the database, rather than reuse the old database after removing the data.