You just need to verify domain test.dom.com in O365, once you do you can add multiple subdomains. So if you have already verified test.dom.com in O365 you can add sub.test.dom.com from 'Domains' option under 'Settings' in Admin Portal and no additional verification or TXT record is required to prove ownership of this subdomain. However you still need to add MX and other records for mailflow and autodiscover to work correctly for this subdomain.
Once you add and verify test.dom.com, you can add sub.test.dom.com. During the domain add process you will be provided with DNS records that you need for mailflow, you just need to add those to your public DNS. Azure AD can manage your domains correctly as long as you set them up correctly and add the required DNS records correctly.