MDL-60923 backup: Added $CFG->backuptempdir

The new setting will allow to host the temporary backup files
into a specific target directory. Defaults to '$CFG->tempdir/backup'.

Calling make_backup_temp_directory() checks that the required sub-directory
will be properly created under the new target directory.

lib/testing/classes/util.php

@@ -813,6 +813,7 @@ abstract class testing_util {
         }
 
         make_temp_directory('');
+        make_backup_temp_directory('');
         make_cache_directory('');
         make_localcache_directory('');
         // Purge all data from the caches. This is required for consistency between tests.