test: move rsa keypairs to fixtures/keys/

Also adds make'd signatures for use in tests of signing/verification.
All of the moved keys can be regenerated at will without breaking tests
now.

PR-URL: https://github.com/nodejs/node/pull/27962


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>