CAA is checked in staging, and you should be able to do your testing there. I just ran a test myself and got the expected urn:ietf:params:acme:error:caa error.
I think CAA results can be cached by the CA for up to 8 hours, though, so if you're doing lots of testing on the same domain name you may want to keep that in mind.
Let's Encrypt doesn't send iodef emails on failed issuance due to CAA. (Though of course, what you actually care about is what the CAs that you don't use do on attempted issuance, since that's what CAA is supposed to mitigate. I don't know how common iodef support is in the industry at large.)