fix printed value of free space, refs #20957

tests/test_backup.py

@@ -60,7 +60,7 @@ def test_backup_space(server):
             print('There is less than 20% of available space for backups')
             return False
         else:
-            print('There is %s%% of free space' % used_perc)
+            print('There is %s%% of free space' % (100 - used_perc))
             return True
     else:
         print('Failed to check backup space')