mark all cells in a nb with a certain flag

hello, is there a way to mark all cells with a flag so that conditional testing can be used?

In `nbdev1`, we had `# all_slow`, for v2, I'm aware of `skip_exec` and `skip_showdoc`. A follow up question in that case would be can the values for these flags be changed when we use `nbdev_test`?

For now, I am manually adding test flags to each and every cell. Is there an alternative to this? @seeM @hamelsmu ?