There is no quick way to fail over a log shipped database to test DR and fail back. Once you make
the database writeable there is no way to begin the log shipping again without resetting up the log shipping.
Something you could try, and I have no idea if it will work would be to stop the SQL Server service in DR. Copy all the database files. Make the database writeable and do the test. Then stop the SQL Service in DR, delete the database files and restore from the copies that you made. When the database restarts it should come back up in recovery.
This may cause the database to come up suspect. I would recommend testing this especially if resetting up your DR database is very costly.