You might want to take a look at this site.
since the test at https://dnssec.vs.uni-due.de appears to be down for a while now, I've been using:
valid: dig +short TXT dnssec-tools.org
invalid: dig +short A good-A.test.dnssec-tools.org
invalid: dig +short AAAA good-AAAA.test.dnssec-tools.org
- there are lots of possible test records on this site.
- there are no conflicts with things like DoH IP addresses.