Hide and document bad_wildcard_dns_certificate.pem #12408
Description
From https://github.com/grpc/grpc-java/pull/12345/files#r2411282796
All certs need documentation on how to create the cert. See testing/src/main/resources/certs/README for how we've documented things previously.
If we're going and making a directory just for sni-testing, then it seems that shouldn't live in testing where our users can use it; that should be in xds/src/test.
We either need to fix this before the next release, or revert the change, because the file is being exposed.
Assigning to Shiva to coordinate the fix.