Revert "test: move all test keys/certs under `test/fixtures/keys/`"

PR-URL: https://github.com/nodejs/node/pull/28083


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Refael Ackermann (רפאל פלחי) <refack@gmail.com>