Workaround: use the other red flag
Until this feature request is implementedA, you can use the following as a workaround: you can raise a different red flag than the one which you previously raised. For example, if you've raised a spam flag on a post, and then need to re-delete it with a red flag, you can raise an R/A flag to delete it again (Note that the post needs to be unlocked prior to re-flagging it). The other way around works as well (spam flag after an R/A flag). Since both Spam and R/A flags carry the same penalties for the user, and are generally treated by the system identically, this should suffice.
So, for reporting to SmokeDetector, as you desired to do, the procedure would be:
Undelete and Unlock the post.
For this step, you could "clear" (also known as "dispute") the existing red flags, but doing so might send the wrong message to the existing flaggers, so manually undeleting and unlocking is preferred, unless you were the only flagger.
Report the post to SmokeDetector.
There are a couple/few userscripts which make reporting posts a bit easier (e.g. Advanced Flagging, and SOCVR's Request Generator) , but you can always post a !!/report <post URL> "An optional reason for reporting in double quotes"
message in Charcoal HQ.
Wait for SmokeDetector to post the report in Charcoal HQ.
Waiting for the report to be posted in chat lets you be sure that SmokeDetector is done getting data from the SE API.
Re-delete the post by raising the red flag which you have not previously raised.
A. Whether this feature request to allow moderators to re-spam flag is worth implementing will depend on how commonly this is needed, especially given the workaround that's available. I've definitely seen moderators wanting to clear red flags for various reasons (usually for testing detections as in your case, or to be able to report a post to SmokeDetector in order to have a record of that spam), but I don't recall seeing a case where that was needed twice. This situation is probably rare enough that it's easiest to just coordinate with another site moderator or a CM when you want to do that.