Thanks, Ric! It was indeed the latter bug that I was thinking of.... But you're probably thinking of this one:
For testing, I'd suggest a procedure like this:
- Clone your system to a separate, external drive (e.g. Samsung T5).
- Password-protect your main drive (i.e. enable FileVault encryption).
- Boot from your external clone drive, making sure not to mount your main drive.
- Install the software you want to test (on this external clone drive), and proceed with testing.
You are absolutely right that the best thing in this case is not just to have a clone, but to work from it. Then simply discard it and re-clone when the testing is done. For some reason, that angle never occurred to me.