feat(tests): create a .nobackup file in the data/database directory

there is no need to backup data/database. Many backup tools use a `.nobackup`
file to prevent from backuping a directory. Let's add one in the data/database
directory.


close #298

cubicweb/devtools/__init__.py

@@ -291,6 +291,9 @@ class TestDataBaseHandler(object):
         except:
             if not isdir(backupdir):
                 raise
+        # Add a .nobackup file in the database directory
+        # so that backup utilities, like Borg, do not backup this directory
+        open(join(backupdir, ".nobackup"), 'w').close()
         return backupdir
 
     def config_path(self, db_id):