Migrate full and incremental backup test functionalities into wtbackup class

From the ticket WT-6866, the backup tests are refactored to use a backup_base class which utilises functions that are similar across different backup tests. The functions take_full_backup, take_incr_backup and range_copy are still left to be migrated into the base class, to complete a common code class.

This ticket is created as a follow up ticket, as the initial ticket was becoming too large.