test: move test_[key|ca|cert] to fixtures/keys/

Lots of changes, but mostly just search/replace of
fixtures.readSync(...) to fixtures.readKey([new key]...)

Benchmarks modified to use fixtures.readKey(...):
benchmark/tls/throughput.js
benchmark/tls/tls-connect.js
benchmark/tls/secure-pair.js

Also be sure to review the change to L16 of
test/parallel/test-crypto-sign-verify.js

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>