When doing a review, I added a comment, then clicked "I'm done". The system said "This was only a test, ...".
Will my comment be posted anyway?
I tested this in all of the review queues for which it applies (that is all review queues except Suggested Edits and Help and Improvement), and for both known good and known bad posts.
In Close Votes, Reopen Votes, Low Quality Posts and Triage, it won't even let you submit your comment. When you try to submit a comment on an audit (either good or bad) in any of those queues, you will get an error saying "this is an audit". Here is an example taken from this audit:
Note that if you comment on an audit after passing or failing it, it will let you post the comment and the comment will be posted on the real post unless the post is deleted or locked.
In First Posts and Late Answers, commenting is considered as an action on the post, and if you comment, it will make you pass the audit if the post is bad and fail the audit if the post is good. The fact that commenting makes you fail a known good audit is a known issue and I (and many others) think that it shouldn't be that way. This question and its many duplicates ask for that to change, but still nothing has changed.
I've tested commenting on a known good and a known bad audit in the First Posts queue, and I've failed and passed those audits respectively (this is the first time I intentionally failed an audit, and luckily I didn't get banned). This was the known good audit I commented on, and as you can see in the original question, my comment wasn't submitted. Here are screenshots of that audit before and after I submitted my comment (I admit that's not the best comment, but it was just for test purposes):
I would guess for known bad audits the comment isn't either submitted, since non-moderators aren't allowed to comment on deleted posts outside of the review queue, but I can't verify it for sure since I don't have the privilege to view deleted posts on any sites. If someone with 10k reputation on Stack Overflow wants to verify this, here is the known bad audit I tested it on.