x509util: Break dependency from ctfe

[pav-kv]

This change removes the dependency of x509util tests from ctfe/testonly
package introduced in #903.

There are 2 certificates copy-pasted between the two packages, but they don't
strictly need to match, and having them independent is a bigger advantage.

Checklist

• I have updated the CHANGELOG.
  • Adjust the draft version number according to semantic versioning rules.
• I have updated documentation accordingly.

[pav-kv]

This change is purely mechanical, it only removes the unneeded dependency. It does not change the exported surface, except removes some now unused constants in ctfe/testonly package.

May review commit-by-commit to follow the thoughts process, or the final state (in which case it can be harder to see the equivalence).