Totally, you can easy do *.test.yourdomain.com and that's works just fine for certbot. Ive never used cloudflare so I'd assume the same setup should work.
Last I checked, which was honestly two or more years prior, CloudFlare doesn’t handle second level sub domains (I.E. a.b.domain.ext) properly… when I tried it, I could make the DNS records, it did resolve, but the certificates didn’t work. I don’t know if that has since changed.
You likely wouldn't be using cloudflare for that level anyways, since you want it to work when you're offline you'd bypass them entirely with local DNS server, local reverse proxy+certs. You'd use something like certbot with let's encrypt which works fine. https://certbot.eff.org/