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>

test/parallel/test-https-unix-socket-self-signed.js

@@ -10,8 +10,8 @@ tmpdir.refresh();
 const fixtures = require('../common/fixtures');
 const https = require('https');
 const options = {
-  cert: fixtures.readSync('test_cert.pem'),
-  key: fixtures.readSync('test_key.pem')
+  cert: fixtures.readKey('rsa_cert.crt'),
+  key: fixtures.readKey('rsa_private.pem')
 };
 
 const server = https.createServer(options, common.mustCall((req, res) => {